Science: every term, defined

Every word defined anywhere in the science courses — 945 of them. Each links to its definition and to the lessons where it is taught, so you can look a word up and go straight to the place it is explained.

A

• abiotic factor — A non-living influence — temperature, water, light, soil, space — which can hold a population well below what its food supply would allow.
• absolute pressure — Pressure measured relative to zero pressure, rather than only above local air pressure.
• absolute zero — The lowest possible temperature, 0 K (about −273°C), where particle motion is at its theoretical minimum.
• absorbed dose — Energy deposited per unit mass, measured in gray.
• absorption — The movement of small food molecules out of the gut and into the blood, mostly across the lining of the small intestine.
• abundance — How many individuals there are, in total or of each species; identical to richness in telling you nothing about distribution.
• acceleration — How quickly velocity changes over time, found by dividing the change in velocity by the time it took.
• accretion — Growth as smaller bodies collide and join.
• acid — A substance that increases hydrogen-ion activity in water; common dilute acidic solutions have pH below 7 near room temperature. Never identify experimental samples by taste.
• acidification — A decrease in pH over a specified comparison.
• acquired characteristic — A change during an organism's life, such as developed muscles or a scar, which does not alter gamete DNA and is therefore not inherited.
• activation energy — The extra energy needed to get a reaction started, even one that will end up releasing energy overall.
• activation energy (Ea) — The minimum energy colliding particles must have for a reaction to occur — the 'hump' reactants must climb before they can become products.
• active site — The precise region of an enzyme where the substrate binds and the reaction takes place.
• active transport — Movement of substances against their concentration gradient, requiring energy (ATP) and carrier proteins.
• activity — Number of decays per second.
• activity series — A ranked list of metals from most to least reactive, used to predict whether a single-replacement reaction will actually happen.
• actual yield — The amount of product actually collected when the reaction is run in the real world.
• adaptation — Reducing exposure or harm under changed conditions.
• ADP — Adenosine diphosphate; it can be converted to ATP using an energy source and phosphate.
• aerobic respiration — Energy-transfer metabolism using oxygen as the final electron acceptor.
• air mass — A huge body of air that takes on a fairly uniform temperature and humidity from the surface it sits over for days at a time.
• air pressure — Force per area associated with air’s interactions and atmospheric conditions.
• air resistance — A type of friction caused by air pushing against a moving object.
• albedo — The fraction of incoming radiation reflected by a surface.
• aliquot — A measured portion of a larger sample.
• alkalinity — A defined measure of acid-neutralizing capacity.
• allele — A version of a gene; an organism with paired chromosomes carries two alleles of most genes, which may be the same or different.
• allele frequency — How common a particular allele is in a population's gene pool, usually written as a fraction or percentage of all copies of that gene.
• alloy — A mixture of a metal with one or more other elements, such as steel or brass.
• alpha decay — Decay in which a nucleus emits an alpha particle (a helium-4 nucleus, ⁴₂He), lowering its mass number by 4 and its atomic number by 2.
• alternative explanation — A rival account consistent with the same evidence; the honest one to name is the strongest, not the easiest to dismiss.
• amino acid — One of the 20 building blocks that link together in a specific order to form a protein.
• ampere (A) — The SI unit of current; one ampere is one coulomb of charge flowing per second.
• amplitude — Largest displacement from rest in the simple oscillation model.
• analog — Continuously varying representation.
• analogous structures — Body parts that perform a similar function and may look alike but do not share a recent common origin — such as the wings of insects and birds — resulting from convergent evolution.
• anion — A negatively charged ion, formed when an atom gains one or more electrons (typically a nonmetal).
• anomaly — A difference from a defined reference value.
• antibody — A protein shaped to match molecules on one particular pathogen; a match for one does not fit another.
• antigen — A feature recognized by adaptive immune receptors.
• antinode — A position with maximal oscillation amplitude.
• antiparallel — The two paired DNA backbones run in opposite 5′/3′ chemical directions.
• apparent brightness — Energy received per unit area per time at an observer.
• aquifer — An underground layer of rock or sediment that holds and transmits groundwater, which can be pumped up through wells.
• area under the curve — The region between a graphed line and the time axis; on a velocity-time graph it equals displacement.
• Arrhenius acid — A substance that releases hydrogen ions (H⁺, which form H₃O⁺) when dissolved in water.
• Arrhenius base — A substance that releases hydroxide ions (OH⁻) when dissolved in water.
• artery — A vessel carrying blood away from the heart, with thick muscular walls for the pressure of each beat — defined by direction, not by oxygen content.
• artificial selection — Selection in which a person rather than the environment decides which individuals reproduce; the mechanism downstream is identical to natural selection.
• asexual reproduction — Production of offspring from one parent with no gamete fusion, giving genetic copies; fast and partner-free, and it generates no variation.
• assumption — Something a result depends on without having established it — a representative sample, complete detection, a full list of pathways; stated beside the result.
• asteroid — A small, rocky object orbiting the Sun, most commonly found in the asteroid belt between Mars and Jupiter.
• asthenosphere — The soft, slowly-flowing mantle layer beneath the lithosphere that the tectonic plates slide on.
• astronomical unit — A distance unit based on the Earth–Sun scale, about 150 million km.
• astronomical unit (AU) — A distance unit equal to the average distance from Earth to the Sun, about 93 million miles (150 million km); used to keep solar system distances easy to compare.
• atom — The smallest unit of an element that still has the properties of that element.
• atomic mass — The average mass of an atom of an element, in atomic mass units, read from the periodic table.
• atomic mass unit (amu) — The standard unit of atomic-scale mass, defined so that one carbon-12 atom has a mass of exactly 12 amu.
• atomic number — The number of protons in an atom; it identifies which element the atom is.
• atomic number (Z) — The number of protons in an atom's nucleus; it defines which element the atom is.
• atomic radius — A measure of the size of an atom, roughly the distance from the nucleus to the outermost electrons.
• ATP — Adenosine triphosphate — the molecule cells use to store and deliver energy for their activities.
• attribution — Evaluation of the causes of an observed change.
• Aufbau principle — Electrons fill the lowest-energy available sublevel first before moving to higher ones.
• autotroph — An organism that builds organic carbon from inorganic sources using light or chemical energy.
• average atomic mass — The weighted average mass of all naturally occurring isotopes of an element, weighted by each isotope's abundance.
• average density — Total mass divided by the total volume enclosed by an object.
• average power — Transferred energy divided by the stated elapsed time.
• average speed — Total distance divided by total time; a scalar that is never negative.
• average velocity — Displacement divided by elapsed time, including direction.
• Avogadro's Law — Equal volumes of gases at the same temperature and pressure contain equal numbers of particles; volume is directly proportional to moles (V ∝ n).
• Avogadro's number — The exact entity count per mole, 6.02214076×10²³; the Avogadro constant carries unit mol⁻¹.
• axis — The imaginary line Earth spins around, running through the North and South Poles.

B

• balanced equation — A chemical equation where the number of each type of atom is the same on both sides.
• balanced forces — Forces that combine to a net force of zero, so motion doesn't change.
• barometer — An instrument that measures air pressure, useful for tracking whether weather is likely to clear or worsen.
• barrier — A defence that prevents entry — unbroken skin, mucus in the airways, tears, stomach acid — each closing a particular route.
• base — A substance that decreases hydrogen-ion activity in water; common dilute basic solutions have pH above 7 near room temperature. Never identify unknown samples by touch or taste.
• baseline — The reference period or value used for comparison.
• beta decay — Decay in which a neutron becomes a proton and emits a beta particle (an electron, ⁰₋₁e), raising the atomic number by 1 while the mass number stays the same.
• binding energy — Energy needed to separate a bound nucleus into its components.
• binomial nomenclature — The two-part Latin naming system, devised by Carolus Linnaeus, that gives every species a unique name from its genus and species — for example, Homo sapiens.
• biocapacity — The capacity of an ecosystem, or the whole planet, to regenerate the resources people use and absorb their wastes in a given year.
• biodiversity — The variety of life in a place, measured at the level of genes within species, species within communities, and habitats within landscapes.
• biogeochemical cycle — The pathway by which an element or molecule moves through living things (bio), the Earth (geo), and chemical reactions — for example, the carbon, nitrogen, and water cycles.
• biogeography — The study of where species live and how they are distributed across the globe, which reflects their evolutionary history and the movement of the continents.
• biomagnification — The increasing concentration of a persistent pollutant in body tissues at each higher step of a food chain, so top predators carry the largest doses.
• biomass — The total mass of living material, a standing stock at one moment rather than a flow over time.
• biome — A large region defined by a characteristic climate and the community of plants and animals adapted to it, such as tundra or tropical rainforest.
• biosphere — Living organisms and their biological relationships within Earth’s systems.
• biotic factor — A living influence on a population — food, predators, disease, competitors.
• block (s, p, d, f) — A region of the table named for the sublevel that is being filled across it, such as the d-block for transition metals.
• boiling point — The temperature at which a liquid turns into a gas throughout the whole liquid.
• bond energy — The energy needed to break one mole of a particular chemical bond; the same amount is released when that bond forms.
• bonding pair — A pair of electrons shared between two atoms, forming a covalent bond.
• Boyle's Law — At constant temperature, the pressure and volume of a fixed amount of gas are inversely proportional: P₁V₁ = P₂V₂.
• Brønsted-Lowry acid — A proton (H⁺) donor in a chemical reaction.
• Brønsted-Lowry base — A proton (H⁺) acceptor in a chemical reaction.
• buffer capacity — Amount of added acid/base a buffer can accommodate effectively.
• buoyancy — The upward force a fluid exerts on an immersed or partly immersed object.
• buoyant force — The net upward force a fluid exerts on an immersed object.

C

• calibration — Checking a measuring system against a known standard before trusting it; every size read from an image assumes its calibration is correct.
• calorimeter heat capacity — Energy needed to raise the apparatus temperature by one degree.
• Calvin cycle — The second, light-independent stage, in the stroma, where ATP and NADPH are used to build sugar from carbon dioxide.
• capacitance — Charge stored per potential difference in a linear model.
• capacity factor — The fraction of the time a power source actually delivers its maximum possible output, written as a percent; intermittent sources have lower capacity factors than steady ones.
• capillary — A vessel one cell thick and barely wider than a red blood cell, where every exchange with tissue actually happens.
• carbohydrate — A macromolecule made of sugars, used for quick energy and structure; its monomer is a monosaccharide (simple sugar).
• carbon cycle — Transfers of carbon among organisms, atmosphere, water, soil, and rocks.
• carrying capacity — The number an environment can sustain given its resources; not a fixed ceiling, since a population can overshoot it and damage the resource.
• carrying capacity (K) — The maximum population size an environment can sustain over the long term, given its resources and limiting factors.
• catalyst — Any substance that speeds up a chemical reaction without being consumed by it.
• cation — A positively charged ion, formed when an atom loses one or more electrons (typically a metal).
• cell — The basic structural and functional unit of life, bounded by a membrane; Hooke named it in 1665 after the small rooms called cellulae.
• cell cycle — The sequence a cell passes through between divisions: a long growth phase including DNA replication, then division, with checkpoints controlling progress.
• cell division — The process by which one existing cell produces new cells; it increases cell number, while growth in mass requires material taken in from outside.
• cell theory — Three connected claims: every living thing is made of one or more cells, the cell is the basic unit of structure and function, and new cells come only from existing cells.
• cell wall — A rigid cellulose layer outside a plant cell's membrane that resists pressure from within; it is porous, so it does no selecting.
• cellular respiration — The release of energy from glucose using oxygen, in mitochondria, giving out carbon dioxide and water; it runs in plant cells as well as animal cells.
• centripetal acceleration — The acceleration of an object moving in a circle, always directed toward the center; a = v²/r.
• centripetal force — The net inward force required to keep an object moving in a circle; F = mv²/r. It is supplied by a real force such as tension, friction, or gravity.
• centromere — A chromosome region associated with chromatid organization and attachment machinery; joined sister units are counted as one chromosome before separation.
• Charles's Law — At constant pressure, the volume of a fixed amount of gas is directly proportional to its Kelvin temperature: V₁/T₁ = V₂/T₂.
• checkpoint — A control process that helps coordinate cell-cycle progression.
• chemical change — A change that forms substances with different chemical identities.
• chemical energy — Energy associated with arrangements and interactions of atoms. Breaking bonds requires energy and forming bonds releases energy; the net balance determines reaction energy transfer.
• chemical equation — A shorthand way of writing a reaction: reactants on the left, an arrow, then products on the right.
• chemical property — A characteristic that describes how a substance reacts or changes into a different substance, like flammability or reactivity with acid.
• chemical reaction — A process where reactants interact and rearrange into one or more new products.
• chlorophyll — The green pigment inside chloroplasts that absorbs light energy — mostly red and blue light — to power photosynthesis.
• chloroplast — The organelle holding chlorophyll, where photosynthesis captures light energy and builds glucose; absent from root cells.
• chromosome — One very long DNA molecule packaged with proteins so it can be handled during division; humans have 46, in 23 pairs.
• circuit — A connected path supporting charge flow and energy transfer under stated conditions.
• circulation — A connected pattern of fluid movement with exchange and return pathways.
• citric acid cycle — A cycle oxidizing acetyl groups, releasing CO₂ and supplying electron carriers in a typical respiration model.
• cladogram — A branching diagram that groups organisms by shared, derived traits to show their evolutionary relationships and common ancestry.
• claim — The statement being asserted — what you think is true — stated precisely enough that evidence could contradict it.
• classification — Organising organisms by shared ancestry, producing a branching tree rather than a ladder from simple to advanced.
• climate — Long-term patterns and distributions of atmospheric conditions in a region.
• climate feedback — A process set off by warming that either amplifies it (a positive feedback, like melting ice) or dampens it (a negative feedback).
• closed system — A system where nothing (no gas, liquid, or solid) can enter or escape, so all mass stays inside to be measured.
• codominance — Both allelic products are detectably expressed in a heterozygote.
• codon — A group of three mRNA bases that codes for one specific amino acid (or a start/stop signal).
• coefficient — The number written in front of a chemical formula, showing how many molecules or units of it are used.
• coefficient of friction (μ) — A number describing how rough a pair of surfaces is; larger μ means more friction. Static μₛ is usually greater than kinetic μₖ.
• coexistence — Presence of two phases together under the stated conditions.
• coil — Wire wound into repeated turns. Current through the turns produces a magnetic field.
• collision — Short interaction in which objects exert forces on each other.
• collision theory — The model that reactions happen only when particles collide with enough energy and the correct orientation.
• combined gas law — A single relationship that merges the three individual laws: P₁V₁/T₁ = P₂V₂/T₂.
• combustion — The chemical reaction of burning a fuel with oxygen, releasing the fuel's stored chemical energy as heat and light and producing carbon dioxide and other emissions.
• combustion reaction — A reaction in which a fuel (usually a hydrocarbon) reacts rapidly with oxygen, releasing energy and typically producing carbon dioxide and water.
• common ancestor — A population from which two lineages diverged; it is represented by a node on a tree and is not any living species.
• community — All the populations of different species living in the same area, interacting with each other.
• compass — An instrument with a magnetized needle; its north-seeking end points along the local magnetic field.
• competition — Two organisms needing the same limited resource, so each does worse than it would alone; the evidence is a comparison, since nothing visible happens between them.
• complementary base pairing — The rule that adenine always pairs with thymine (A-T) and cytosine always pairs with guanine (C-G).
• component — The projection of a vector onto a chosen axis.
• compression — Reduction of the space occupied by a material.
• concentration — The amount of a substance in a given volume; it only supports a prediction when stated as a comparison between two places.
• condensation — Vapour returning to liquid as air cools, forming cloud; a change of state rather than a chemical reaction.
• conduction — Heat transfer through direct contact, particle to particle.
• conductor — A material in which charge can move relatively readily under stated conditions.
• confound — A second factor that varied alongside the independent variable, so a difference cannot be attributed to either one.
• confounding variable — Another changing factor that could explain the measured outcome.
• conjugate pair — Species that differ by one proton.
• conservation — Protecting or responsibly managing resources and ecological relationships through defined actions.
• conservation of energy — Energy remains accounted for across a complete system and its transfers.
• conservation of mass — Matter remains accounted for when the complete relevant system and exchanges are included.
• conservation of mechanical energy — In the absence of friction and other non-conservative forces, KE + PE stays constant: KE_i + PE_i = KE_f + PE_f.
• conservation of momentum — When no net external force acts, the total momentum of a system is the same before and after an interaction.
• constraint — A limit that the proposed solution must satisfy.
• consumer — An organism obtaining energy by eating others; the role describes a relationship rather than labelling a species permanently.
• continental drift — Alfred Wegener's idea that continents were once joined together and have since slowly moved to their current positions.
• contour — A line connecting locations of equal elevation.
• contour interval — The elevation difference between successive contour values.
• contraction — The shortening of a muscle that generates force; relaxing produces no push, only the possibility of being stretched by a partner.
• controlled variable — A factor deliberately held the same across all groups so it cannot explain any difference found.
• convection — Energy transport associated with bulk motion of a liquid or gas.
• convergent boundary — A place where two tectonic plates move toward each other and collide.
• correlation — Two quantities changing together — a real finding, but not on its own evidence that one causes the other.
• cosmic microwave background — Radiation observed across the sky left from an early stage of the universe.
• cost effectiveness — Benefit per unit of cost, which can rank options differently from total benefit — 0.15 against 0.167 litres per token here.
• Coulomb's law — The rule that the force between two charges is proportional to the product of the charges and inversely proportional to the square of the distance: F = kq₁q₂/r².
• covalent bond — A chemical bond in which two atoms share a pair of electrons.
• criterion — A stated measure of desired success in a design problem.
• cross-cutting relationship — The principle that a fault or rock intrusion cutting across existing rock layers must be younger than the layers it cuts through.
• cross-link — A bond connecting polymer chains into a network.
• crossing over — The exchange of matching segments between homologous chromosomes during meiosis, creating new combinations of alleles.
• crust — Earth's thin, rocky outer layer — oceanic crust (about 5–10 km thick, basalt) under the oceans, and thicker continental crust (about 30–50 km thick, granite) under land.
• crystal lattice — The orderly, repeating three-dimensional arrangement of ions in an ionic solid.
• current — The rate of electric-charge flow through a chosen part of a circuit, measured in amperes (A).
• cytokinesis — The final step of division, in which the cytoplasm splits to form two separate daughter cells.
• cytoplasm — The jelly-like material filling the cell inside its membrane, holding the organelles in place.

D

• data — The recorded measurements and observations themselves, kept raw so that any summary drawn from them can be checked.
• daughter cell — Either of the two cells produced, each with the same chromosome number as the original.
• daughter nucleus — Nucleus produced by radioactive decay.
• daughter product — A product formed through radioactive decay.
• decay constant — Probability-rate parameter linking remaining nuclei to activity.
• decomposer — An organism breaking down dead material and waste, returning carbon and nutrients to forms producers can use again.
• decomposition — Biological breakdown and transformation of organic material into other matter stores and forms.
• decomposition reaction — A reaction that breaks a larger molecule into smaller ones — a breaking-down reaction.
• deformation — Change in an object’s shape.
• delocalized electrons — Valence electrons that are not tied to a single atom but move freely throughout a piece of metal.
• denature — To lose the normal 3-D shape (and therefore function), as when a protein is exposed to high heat or the wrong pH.
• denitrification — Microbial pathways converting oxidized nitrogen compounds to gaseous forms under suitable conditions.
• density — Mass per unit volume under stated conditions.
• dependent variable — The response measured under the stated method.
• deposition — The settling and dropping of transported sediment when the agent carrying it — water, wind, or ice — loses energy and slows down.
• derived feature — A feature changed from the ancestral state used for comparison.
• development — Changes that form and organize an organism across its life cycle.
• dew point — Temperature at which cooling air reaches saturation under the specified conditions.
• diatomic molecule — A molecule made of exactly two atoms, such as O₂ or N₂.
• differentiation — Separation of materials into layers, often by density.
• diffusion — The net movement of particles from a crowded region to a sparse one, produced by random motion alone and costing the cell nothing.
• diffusion distance — The path length over which particles must move.
• digestion — Breaking food down, mechanically into smaller pieces and chemically into smaller molecules; only the chemical stage makes molecules small enough to absorb.
• digital — Representation using discrete permitted symbols.
• dilution — Adding solvent to a solution to lower its concentration; the moles of solute stay the same, so M₁V₁ = M₂V₂.
• dimension — The physical kind of a quantity, such as length or time.
• diploid — Having chromosomes in pairs — 46 in 23 pairs for humans — with one member of each pair originally from each parent.
• dipole — A separation of partial positive and partial negative charge across a bond or a whole molecule.
• dipole-dipole forces — Attractions between the permanently positive end of one polar molecule and the negative end of another.
• dispersal — Moving seeds away from the parent by borrowing movement from wind, water or animals, so offspring do not compete with an established plant.
• displacement — The fluid volume moved aside by a submerged object.
• dissipation — Energy spreading into less readily useful forms or surroundings for the chosen purpose.
• dissolved inorganic carbon — The pool of dissolved CO₂, bicarbonate, carbonate, and related inorganic carbon species.
• distance — The total length of the path an object travels; a scalar, always positive, with no direction.
• disturbance — An event disrupting a community — fire, flood, drought, felling; many communities are shaped by regular disturbance rather than harmed by it.
• divergent boundary — A place where two tectonic plates move apart, allowing magma to rise and form new crust, as at a mid-ocean ridge.
• DNA — The long molecule whose sequence carries genetic information; about two metres of it is packed into a nucleus six micrometres across.
• DNA polymerase — The enzyme that builds a new DNA strand by adding matching nucleotides one at a time along a template strand.
• domain — The broadest, highest level of classification; all life is divided into three domains — Bacteria, Archaea, and Eukarya.
• dominance — The stated rule relating genotype to appearance; under complete dominance one copy of the dominant allele is enough to produce its effect.
• dominant — Expressed in the heterozygote under a specified complete-dominance model; not necessarily common or beneficial.
• Doppler effect — The change in observed frequency (pitch) when a wave source and observer move relative to each other.
• Doppler shift — Change in observed frequency associated with relative motion.
• double bond — A covalent bond in which two pairs of electrons (four electrons) are shared.
• double helix — The twisted-ladder shape of a DNA molecule, formed by two strands winding around each other.
• double-replacement reaction — A reaction where two compounds swap partners, exchanging ions (AB + CD → AD + CB).
• dry mass — Mass with the water driven off, used because water content varies daily and would otherwise swamp the change being measured.
• ductile — Able to be drawn out into a wire.
• dynamic equilibrium — The state in which the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products stay constant.

E

• eclipse — An alignment event in which one body blocks light or another enters a shadow.
• ecological efficiency (10% rule) — The general finding that only about 10% of the energy at one trophic level is stored as biomass the next level can eat; the rest is used or lost as heat.
• ecological footprint — A measure of how much productive land and water area a person or population requires to supply what it consumes and absorb the waste it produces.
• ecosystem — A community of organisms together with their physical surroundings, connected closely enough that a change in one part appears in others.
• ecosystem diversity — The variety of different ecosystems, habitats, and ecological communities across a region.
• ecosystem services — The benefits people get from functioning ecosystems, grouped as provisioning, regulating, supporting, and cultural services.
• effective collision — A collision that actually leads to a reaction because it has sufficient energy and proper orientation.
• effective nuclear charge (Zeff) — The net positive pull actually felt by a valence electron, after inner electrons cancel out some of the nuclear charge.
• effector — A muscle, gland, or organ that carries out the response that restores balance.
• efficiency — Useful output energy divided by input energy, often expressed as a percentage.
• elastic collision — A collision in which both momentum and kinetic energy are conserved.
• elastic deformation — Shape change that substantially reverses after the force is removed.
• elastic potential energy — Energy stored in a stretched or compressed elastic object such as a spring: PE = ½kx².
• electric charge — A property responsible for electric interactions. In this introductory model, excess electrons give an object net negative charge and an electron deficit can give it net positive charge.
• electric current — The rate at which electric charge flows past a point, I = Q/t, measured in amperes (A).
• electric field — The region around a charge where another charge would feel a force; its strength is the force per unit charge, E = F/q, in N/C.
• electric motor — A device that uses the force on a current-carrying wire in a magnetic field to convert electrical energy into motion.
• electric potential — Electric potential energy per unit charge.
• electrical energy — Energy carried by moving electric charge through a circuit.
• electrical power — The rate at which a device converts electrical energy, P = IV, measured in watts (W).
• electrolysis — An electrically driven chemical change.
• electrolyte — A substance producing mobile ions in solution or melt.
• electromagnet — A magnet produced by electric current, often using a wire coil and an iron core.
• electromagnetic (EM) wave — A wave that can travel through empty space, carrying energy as vibrating electric and magnetic fields.
• electromagnetic induction — The production of a voltage (and current) in a conductor by a changing magnetic field.
• electromagnetic spectrum — The full range of EM waves, ordered from long, low-energy radio waves to short, high-energy gamma rays.
• electron — A negatively charged particle that moves around the nucleus in an electron cloud.
• electron configuration — The notation showing how many electrons occupy each sublevel of an atom, such as 1 s² 2 s² 2p⁶.
• electron sea — The model that pictures a metal's valence electrons as a mobile 'sea' shared by all of the metal ions at once.
• electronegativity — A measure of how strongly an atom attracts the shared electrons in a chemical bond.
• electronvolt — Energy transferred to one elementary charge across 1 V.
• element — A pure substance made of only one kind of atom, such as carbon, oxygen, or hydrogen.
• embodied energy — Energy used before a product’s operating life within a stated boundary.
• embryo — The tiny plant inside a seed, with the beginnings of a root and shoot, packed alongside a food store.
• emf — Source energy per charge under the modeled open-circuit condition.
• empirical formula — Simplest whole-number atom ratio.
• endocytosis — Bulk transport in which the membrane wraps around material and pulls it into the cell inside a vesicle.
• endoplasmic reticulum — A folded network of membranes that processes proteins (rough ER, studded with ribosomes) and makes lipids (smooth ER).
• endothermic reaction — A reaction that absorbs energy from its surroundings, usually cooling them down.
• endpoint — Observed signal used to stop a titration.
• energy — The ability to do work or cause change, measured in joules (J).
• energy budget — A full account of where energy goes, including respiration as heat, uneaten parts, waste and decomposers — the routes a pyramid leaves out.
• energy coupling — Linking reactions or processes so a favorable change can support work.
• energy dissipation — Energy spreading into less useful forms, often thermal energy in the surroundings.
• energy level (n) — A principal shell around the nucleus; higher n means higher energy and, on average, greater distance from the nucleus.
• energy pyramid — A diagram of the energy passing through each level over a stated period; unlike a biomass pyramid, it can never invert.
• energy transfer — Energy moving from one object or place to another without necessarily changing form.
• energy transformation — A change of energy from one form into another, such as chemical energy becoming electrical energy.
• enthalpy change (ΔH) — The heat absorbed or released by a reaction at constant pressure; negative for reactions that release heat, positive for reactions that absorb it.
• entropy — A measure related to accessible microscopic arrangements and energy dispersal.
• environmental influence — A factor in the surroundings — light, nutrition, temperature, injury, learning — affecting how genetic information is expressed.
• enzyme — A protein that cuts a particular kind of molecule — amylase on starch, protease on protein, lipase on fat — working within a range of acidity and temperature.
• eon — The largest unit of geologic time; Earth's history is divided into a few eons, with the Phanerozoic being the most recent.
• epicenter — The point on Earth's surface located directly above the focus.
• equilibrium — The state where two opposite flows match, so the net movement is zero; the particles are still moving and still crossing.
• equilibrium constant (Keq) — A number equal to the product concentrations divided by the reactant concentrations (each raised to its coefficient) that shows whether products or reactants are favored.
• equilibrium shift — A temporary imbalance between the forward and reverse rates that moves the system to a new equilibrium position.
• equinox — One of two days a year (around March 20 and September 22) when Earth's tilt is sideways to the Sun, so day and night are roughly equal length nearly everywhere.
• equipotential — A surface or set of points at equal electric potential.
• equivalence — Reactants present in the stoichiometric proportions for the selected reaction.
• equivalence point — Stoichiometric completion for the selected titration reaction.
• equivalent resistance — The single resistance that could replace a whole combination of resistors and draw the same current.
• era — A division of geologic time within an eon — the Phanerozoic eon, for example, is split into the Paleozoic, Mesozoic, and Cenozoic eras.
• erosion — Transport of material by agents such as flowing water, wind, ice, or gravity.
• estuary — A partly enclosed coastal area where fresh river water mixes with salt water, creating brackish, nutrient-rich, highly productive habitat.
• eukaryote — A cell with a true nucleus and membrane-bound organelles that compartmentalize its work. Animals, plants, fungi, and protists are eukaryotes.
• eukaryotic — Having DNA enclosed in a nucleus, with membrane-bound organelles; animals, plants, fungi and protists.
• eutrophication — The over-enrichment of water with nutrients (often from fertilizer runoff) that fuels algal blooms and, when the algae decompose, creates oxygen-starved 'dead zones.'
• evaporation — A change from liquid to gas at a surface, possible below boiling.
• evenness — How individuals are spread among the species; 8,1,1 and 4,3,3 have the same richness and abundance and very different evenness.
• evidence — A relevant observation, measurement, or record used to evaluate a claim.
• excess reactant — A reactant that is left over after the limiting reactant is completely used up.
• excited state — An arrangement with energy above the ground state.
• excretion — Removing waste made by the body's own metabolism — carbon dioxide from respiration, urea from surplus amino acids — which is not the same as passing undigested food out.
• exocytosis — Bulk transport in which a vesicle fuses with the membrane to release its contents outside the cell.
• exothermic reaction — A reaction that releases energy to its surroundings, usually warming them up.
• exponential growth — Growth in which a population increases by a constant proportion each period, producing a rapidly steepening J-shaped curve.
• exposure — Contact between a hazard and people or ecosystems.
• external impulse — Impulse from forces exerted by objects outside the chosen system.
• extinction — A species ceasing to exist; claiming it from one site confuses absence of detection with absence of the species.
• extrapolation — Predicting beyond the range of the data collected; always a guess, and least reliable where the pattern is most likely to change.

F

• facilitated diffusion — Passive transport in which channel or carrier proteins help specific molecules (like glucose or ions) cross the membrane.
• Faraday constant — Charge per mole of electrons, about 96485 C/mol.
• feedback — Using the output of a system as information that changes the system's behaviour; the loop closes when the response alters what the detector senses.
• fermentation — A pathway regenerating NAD⁺ to sustain glycolysis without adding further ATP.
• fertilization — The fusion of two gametes, one event at one moment, which is separate from the development that follows.
• field — A model describing the influence an object has in the space around it.
• filtrate — The liquid and dissolved substances that pass through a filter.
• filtration — Forcing water and small dissolved molecules out of the blood under pressure, holding back cells and large proteins by size alone; nothing is selected at this stage.
• fitness — Reproductive contribution — surviving offspring left — rather than strength or health.
• fluid — A liquid or gas that flows.
• fluid mosaic model — The accepted model of the membrane as a fluid sheet in which proteins and other molecules drift about like tiles in a moving mosaic.
• flux — A rate of transfer between reservoirs.
• focal length (f) — The distance from a lens or mirror to the point where parallel rays converge (or appear to diverge from).
• focus (hypocenter) — The point underground where an earthquake's rupture begins and energy is first released.
• foliation — The parallel banding or wavy layering seen in some metamorphic rocks, created when pressure lines up minerals in the same direction.
• food web — A model of connected feeding relationships among organisms.
• force diagram — Representation of the forces acting on one chosen object.
• formal charge — An electron-counting assignment, distinct from partial charge.
• formula unit — The simplest whole-number ratio of ions in an ionic compound (for example, NaCl).
• fossil — Remains or traces of past life preserved in rock through a rare sequence of events, so the record samples what was preserved rather than what lived.
• fossil fuel — A nonrenewable energy resource — coal, oil, or natural gas — formed from the buried remains of ancient organisms transformed by heat and pressure over millions of years.
• fossil record — The whole body of fossil evidence — biased towards hard parts, towards places where sediment accumulates, and towards places geologists have searched.
• fractional abundance — A percent abundance written as a decimal (percent abundance ÷ 100); it is what you actually multiply by in the average.
• frame of reference — The point of view — a chosen origin and coordinate system — from which position and motion are measured.
• free fall — Motion under the influence of gravity alone, with air resistance neglected.
• free-body diagram — A simplified sketch of a single object with an arrow drawn for every force acting on it, pointed in the correct direction and sized to show relative strength.
• frequency — The proportion of a population carrying a variant; a count without its denominator cannot be compared.
• frequency (f) — The number of waves passing a point each second, measured in hertz (Hz).
• freshwater — Water with very little dissolved salt, usable for drinking and agriculture; it makes up only a small fraction of all the water on Earth.
• friction — A force that opposes motion between two surfaces in contact.
• front — A boundary region between air masses with contrasting properties.
• fulcrum — The pivot about which a lever turns.
• functional group — A structural group associated with characteristic chemical behavior.
• functional unit — The equal service used as a basis for comparison.
• fusion — The joining of light nuclei into heavier nuclei, with energy release in appropriate reactions.

G

• g — The acceleration due to gravity near Earth's surface, about 9.8 m/s² directed downward, the same for every freely falling object regardless of mass.
• G₁, S and G₂ — Typical interphase stages: growth/preparation before copying, DNA synthesis, and preparation after copying.
• galvanic cell — A cell using a favorable reaction to supply electrical energy.
• gamete — A sex cell carrying one complete set of chromosomes, so that fusion restores the diploid number rather than doubling it.
• gamma decay — The release of a high-energy gamma ray (electromagnetic energy) from a nucleus, with no change in mass number or atomic number.
• gas — A state of matter with no fixed shape or volume; particles are far apart and move freely and quickly.
• gas exchange — Diffusion of oxygen into the blood and carbon dioxide out of it across a thin, moist surface with a maintained concentration difference.
• gas giant — One of the two huge outer planets, Jupiter or Saturn, made mostly of hydrogen and helium gas.
• gas pressure — Force per unit area from gas impacts on a surface.
• Gay-Lussac's Law — At constant volume, the pressure of a fixed amount of gas is directly proportional to its Kelvin temperature: P₁/T₁ = P₂/T₂.
• gene — A section of DNA specifying a product, usually a protein, whose shape determines what it can do.
• gene expression — Use of gene information to produce functional RNA or protein.
• gene flow — The movement of alleles between populations when individuals (or their gametes, like pollen) migrate and interbreed, which tends to make populations more genetically similar.
• gene pool — The total genetic variation in a population; breeding from a few individuals narrows it across the whole genome, not only at the selected gene.
• generator — A device that uses electromagnetic induction to convert mechanical motion into electrical energy — essentially a motor run in reverse.
• genetic diversity — The variety of genes and alleles within a population or species, which supplies the raw material for adapting to change.
• genetic drift — Random, chance changes in allele frequencies from one generation to the next; its effect is strongest in small populations, as in the bottleneck and founder effects.
• genetic engineering — Intentional modification or introduction of genetic material using biotechnology.
• genotype — The pair of alleles an organism carries for a gene, written as two letters such as Aa.
• geologic record — Evidence preserved in rocks, minerals, structures, and other Earth materials.
• geologic time scale — The timeline scientists use to divide Earth's 4.6-billion-year history into nested divisions, from largest (eons) down to eras and periods.
• geosphere — Earth’s rocky material, landforms, and interior in a system model.
• germination — The resumption of growth when a seed takes up water and mobilises its food store; the root emerges before the shoot.
• Gibbs free energy — A thermodynamic potential useful for constant-temperature, constant-pressure processes.
• given — The substance whose amount you know at the start of a stoichiometry problem.
• global warming — The rise in Earth's average surface temperature over the past century-plus, linked to human-caused increases in greenhouse gases.
• glycolysis — A cytosolic pathway converting glucose to pyruvate with a net gain of two ATP per glucose.
• Golgi apparatus — A stack of membrane pouches that modifies, sorts, packages, and ships proteins and other products in vesicles.
• gradient — Change in elevation divided by horizontal distance.
• gravitational constant (G) — The fixed number in the gravitation equation, G = 6.67 × 10⁻¹¹ N·m²/kg², measured first by Henry Cavendish.
• gravitational potential energy — Stored energy an object has because of its height in a gravitational field: PE = mgh.
• gravity — An attractive force between any two objects with mass; bigger masses and closer distances create a stronger pull.
• greenhouse effect — The natural process by which greenhouse gases trap some of Earth's outgoing heat, keeping the lower atmosphere warm enough for life.
• greenhouse gas — A gas, such as water vapor, carbon dioxide, or methane, that absorbs and re-emits heat radiating from Earth's surface.
• gross primary productivity (GPP) — The total amount of energy producers capture and fix into organic matter over a given time and area.
• ground state — The lowest-energy available electron arrangement.
• group (family) — A vertical column showing related property patterns. Main-group elements often share an outer-electron count, with qualifications for helium and transition elements.

H

• H–R diagram — A diagram comparing stellar luminosity and surface temperature or spectral class.
• Haber process — The industrial reaction N₂ + 3H₂ ⇌ 2NH₃, used to manufacture ammonia for fertilizer.
• habitat — The setting and conditions an organism uses to live and obtain resources.
• habitat fragmentation — Splitting continuous habitat into pieces; total area can stay the same while patches become too small, disconnected, and mostly edge.
• half-life — Time over which half of a large radioactive parent population remains statistically; not an individual atom’s scheduled decay time.
• haploid — Having one chromosome from each pair: one complete set, not merely half the number.
• hardness — Resistance to scratching under a stated comparative test.
• heat — Energy transfer caused by a temperature difference.
• heat capacity — The energy required to change a substance’s temperature by a specified amount.
• heat of fusion — The energy absorbed to melt a given amount of a solid (and released when that amount freezes).
• heat of vaporization — The energy absorbed to boil a given amount of a liquid (and released when that amount condenses); larger than the heat of fusion.
• heating curve — Graph of temperature during energy input.
• heritability evidence — A comparison in matched conditions, ideally of offspring, showing that an advantage is transmitted rather than produced by circumstances.
• heritable variation — Differences between individuals that can be passed to offspring, supplied by mutation, meiosis and recombination before any pressure arrives.
• Hess’s law — Reaction enthalpies add when equations add.
• heterotroph — An organism that obtains organic carbon from other sources.
• heterozygous — Carrying two different alleles of a gene, such as Aa; such a parent produces two kinds of gamete in equal proportions.
• homeostasis — Keeping internal conditions stable enough for the chemistry of life to continue; the characteristic a fire or a crystal most clearly lacks.
• homologous chromosomes — A matching pair of chromosomes, one inherited from each parent, that carry the same genes.
• homologous structure — A feature sharing an underlying plan through inheritance, such as the one-bone, two-bone, digits forelimb across mammals doing quite different jobs.
• homologous structures — Body parts that share the same underlying anatomical plan because they were inherited from a common ancestor, even when they now perform different functions — such as the forelimbs of humans, whales, and bats.
• homozygous — Carrying two identical alleles, such as AA or aa; such a parent produces only one kind of gamete.
• horizontal component — The part of a projectile's motion parallel to the ground; its velocity here stays constant.
• hormone — A chemical messenger released into the blood by a gland; slower than a nerve signal and carried everywhere.
• hot spot — An area of unusually intense volcanic activity above a rising plume of hot mantle rock, active even in the middle of a plate, far from any boundary.
• humidity — Water vapor content of air, expressed with a specified measure.
• Hund's rule — Within a sublevel, electrons occupy separate orbitals singly before any orbital is doubled up.
• hydraulic fracturing (fracking) — A drilling method that injects high-pressure fluid underground to crack open shale rock and release the oil or natural gas trapped inside it.
• hydrogen bond — A weak attraction between a slightly positive hydrogen in one molecule and a slightly negative atom in another.
• hydrogen bonding — An especially strong dipole-dipole attraction that forms when hydrogen is bonded to N, O, or F and is drawn to a lone pair on the N, O, or F of a neighboring molecule.
• hydrophilic — 'Water-loving' — a part of a molecule that is attracted to water. The phosphate head is hydrophilic.
• hydrophobic — Describes a region that interacts poorly with water and tends to cluster away from water in an aqueous environment. It does not literally fear water or exert a special repelling force.

I

• ICE table — A table tracking initial values, stoichiometric change, and equilibrium values.
• ideal gas — A hypothetical gas that perfectly obeys the KMT assumptions; real gases behave most ideally at low pressure and high temperature.
• ideal gas constant (R) — The proportionality constant in PV = nRT; equal to 0.0821 L·atm/(mol·K), or 8.314 J/(mol·K) in SI units.
• ideal gas law — The equation PV = nRT, which relates the pressure, volume, moles, and Kelvin temperature of a gas in a single snapshot.
• ideal machine — A model with no dissipative energy losses.
• igneous rock — Rock that forms when molten magma or lava cools and hardens (crystallizes).
• immune memory — Cells persisting after recovery that respond to the same pathogen in hours rather than days, which is why some diseases are usually once-only.
• impervious surface — A roof, road or compacted ground that prevents infiltration, sending water to runoff within minutes rather than hours.
• impulse — The product of force and the time it acts, J = FΔt; it equals the change in momentum, Δp.
• incomplete dominance — A heterozygous phenotype is intermediate in the stated model.
• independence of motion — The principle that a projectile's horizontal and vertical motions occur separately and do not affect each other's timing.
• independent variable — The factor deliberately changed for the intended comparison.
• index fossil — The remains of a species that lived for a relatively short span of geologic time but spread across a wide area, making it useful for matching the ages of rock layers on different continents.
• index of refraction (n) — How much a medium slows light: n = c ÷ v, where v is light's speed in the medium; larger n means slower light and more bending.
• indicator — A chemical, like litmus paper, that changes color depending on the pH of a solution.
• indirect evidence — Observations used to infer a feature that is not directly inspected.
• inelastic collision — A collision in which momentum is conserved but some kinetic energy is lost; in a perfectly inelastic collision the objects stick together.
• inertia — The tendency of an object to resist a change in its motion.
• inertial frame — A reference frame where an unforced object has constant velocity.
• inference — A reasoned interpretation of observations rather than the observation itself.
• infiltration — Water entering spaces in soil or other ground material.
• inherited trait — A characteristic whose variation is influenced by genetic information passed from parents.
• inhibitor — A substance that slows a reaction down, the opposite of a catalyst.
• input force — The force applied to a machine.
• inquiry report — An account stating a bounded question, the evidence with units and conditions, reasoning through a mechanism, and the assumptions and limits.
• instantaneous velocity — The velocity at a single instant — the value on a speedometer paired with a direction; the slope of a position-time graph at that point.
• insulator — A material such as rubber or glass whose electrons are locked in place, so charge does not flow through it.
• intensity — The shaking effects experienced at a location.
• interaction — Mutual influence between named objects.
• interference — The result of two or more waves overlapping, adding together to reinforce (constructive) or cancel (destructive).
• intermittency — The tendency of some renewable sources — especially solar and wind — to produce power only when the sun is shining or the wind is blowing, rather than on demand.
• intermolecular force — An attraction between separate molecules; these are much weaker than the bonds within a molecule.
• intermolecular forces (IMFs) — The attractions between separate molecules, as opposed to the much stronger bonds within a molecule.
• internal resistance — A simplified source resistance causing voltage drop and heating.
• interphase — The long part of the cell cycle before division, when a cell grows, does its work, and copies its DNA.
• invasive species — A non-native species that spreads rapidly in a new ecosystem because it lacks the predators, diseases, or competitors that limited it back home.
• ion — An atom or group of atoms with net electric charge because positive and negative charges do not balance.
• ionic bond — Electrostatic attraction between oppositely charged ions. Electron transfer can produce ions but is distinct from the attraction.
• ionization energy — The energy required to remove the most loosely held electron from a neutral atom.
• irrigation — Supplying water to crops artificially through channels, sprinklers, or drip systems; it is the single largest human use of freshwater.
• isobar — A line on a weather map connecting points that all have the same air pressure; closely packed isobars mean stronger winds.
• isotope — An atom of an element distinguished by neutron count; isotopes share proton count.
• isotopes — Atoms of the same element (same number of protons) that have different numbers of neutrons, and therefore different mass numbers.
• iteration — The cycle of testing, learning and changing; a design that has not been through one has not been tested.

J

• joint — A place where bones meet and movement is possible; its shape sets what movement is allowed — a hinge at the elbow, a ball and socket at the shoulder.
• joule — The unit of energy and work; 1 J = 1 N·m.
• joule (J) — The standard unit of energy.

K

• Ka — Acid-ionization equilibrium constant for a specified temperature.
• kelvin (K) — Absolute temperature unit used on the H–R diagram; about 273.15 K corresponds to 0 °C.
• Kelvin scale — An absolute temperature scale whose zero point (0 K) is absolute zero; K = °C + 273.15.
• keystone species — A species whose effect on its ecosystem is far larger than its abundance would suggest, so its loss triggers disproportionate change.
• kilowatt — One thousand watts.
• kilowatt-hour — Energy transferred by an average power of one kilowatt over one hour.
• kinematic equations — A set of four equations linking displacement, initial and final velocity, acceleration, and time for motion with constant acceleration.
• kinetic energy — Energy associated with motion.
• kinetic friction — The friction force between two surfaces that are already sliding past each other; f = μₖN.
• kinetic molecular theory (KMT) — A model that explains the behavior of gases by picturing them as huge numbers of tiny particles in constant, random, straight-line motion.

L

• latent energy transfer — Energy absorbed or released during a change of phase.
• latent heat — Energy per amount of substance associated with a phase change at nearly steady temperature.
• law of conservation of energy — Energy is conserved when stored changes and all transfers across the chosen boundary are counted. An isolated system exchanges neither matter nor energy; a system closed to matter can still exchange energy.
• law of conservation of mass — In a chemical reaction, the total mass of the reactants equals the total mass of the products.
• Le Châtelier's principle — If a system at equilibrium is disturbed, it shifts in the direction that partially counteracts the disturbance.
• lever — A rigid bar turning about a pivot; limbs trade force for speed and range by attaching the muscle close to the joint.
• lever arm — Perpendicular distance from pivot to force line.
• Lewis structure — A diagram that shows an atom's or molecule's valence electrons as dots and each bonding pair as a line.
• life-cycle perspective — Considering impacts across production, use, and end-of-life stages.
• light-dependent reactions — The first stage of photosynthesis, in the thylakoid membranes, where light splits water, releases oxygen, and makes ATP and NADPH.
• light-year — Distance light travels in one year; it is not the age of an object.
• limitation — A respect in which a model or an investigation does not match reality; a good statement names its own.
• limiting factor — Any resource or condition — food, water, space, predators, disease, weather — that restricts how large a population can grow.
• limiting reactant — The reactant that runs out first, capping the amount of product a reaction can make.
• lipid — A broad group of largely hydrophobic molecules, including fats, phospholipids, and steroids; they are not all repeating-unit polymers.
• liquid — A state of matter with a fixed volume but no fixed shape; particles are close but can flow past each other.
• lithification — The combined process of compaction (squeezing sediment together) and cementation (gluing sediment grains with natural mineral 'glue') that turns loose sediment into solid sedimentary rock.
• lithosphere — The rigid outer layer including the crust and uppermost mantle.
• logarithmic scale — A scale whose equal tick steps represent equal multiplication factors; here luminosity ticks change by a factor of ten.
• logistic growth — Growth that starts fast but slows and levels off as a population nears the limits of its environment, producing an S-shaped curve.
• London dispersion forces — Weak attractions present in ALL molecules, caused by fleeting, temporary dipoles from shifting electrons; stronger in larger molecules with more electrons.
• lone pair — A pair of valence electrons on an atom that is not shared in a bond.
• longitudinal wave — A wave where particles vibrate back and forth in the SAME direction the wave travels.
• Lorentz factor — γ=1/√(1−v²/c²), relating measurements in inertial frames.
• loudness — How intense a sound seems; determined by the wave's amplitude and measured on the logarithmic decibel (dB) scale.
• luminosity — Total energy emitted per unit time.
• luster — How a mineral's surface reflects light — described as metallic (shiny like metal) or non-metallic (glassy, pearly, dull, or earthy).

M

• Ma — Million years before present in a stated geological age; larger values are older.
• macromolecule — A large molecule; many biological macromolecules are polymers, but not every biological molecule grouped with them is a repeating-unit polymer.
• magma — Molten (melted) rock beneath Earth's surface; the same material is called lava once it erupts above the surface.
• magnetic field — A representation of magnetic interaction around magnets or electric currents.
• magnification — How many times larger the image is than the object; on a compound microscope, eyepiece multiplied by objective.
• main sequence — The stage in which a star sustains hydrogen fusion in its core.
• malleable — Able to be hammered or rolled into thin sheets without breaking.
• mantle — The thick, mostly solid layer of hot silicate rock between the crust and the core, about 2,900 km deep, that flows extremely slowly over millions of years.
• mass — The amount of matter in an object, measured in kilograms; it doesn't change no matter where the object is.
• mass defect — Difference between separated-component and bound-system rest mass.
• mass number — The total number of protons plus neutrons in an atom's nucleus.
• mass number (A) — The total count of protons plus neutrons in an atom's nucleus.
• mass spectrometer — An instrument that sorts atoms by mass and measures the relative abundance of each isotope.
• mass-to-mass calculation — Finding the mass of one substance from the mass of another by converting to moles, applying the mole ratio, and converting back to mass.
• matter cycling — Reuse and transfer of atoms among organisms and environment.
• mean — The sum of comparable values divided by their count.
• mechanical advantage — Output force divided by input force.
• mechanical energy — The combined kinetic and potential energy of an object.
• medium — The matter (solid, liquid, or gas) that a wave travels through.
• megaparsec (Mpc) — A distance unit of about 3.26 million light-years, used for galaxy separations.
• meiosis — The division producing gametes: one replication followed by two divisions, giving four haploid cells that differ from the parent and from each other.
• melting point — The temperature at which a solid turns into a liquid.
• membrane — Thin surface capable of moving or vibrating.
• membrane protein — A protein embedded in or attached to the membrane that transports materials, receives signals, or identifies the cell.
• metabolism — The chemical reactions going on inside an organism; excretion removes their products, which is why material that merely passed through the gut does not count.
• metal — An element that is typically shiny, malleable, and a good conductor; metals fill the left and center of the table.
• metallic bond — The attraction between positively charged metal ions and the surrounding sea of delocalized electrons.
• metalloid — An element with properties between metals and nonmetals, found along the zigzag line on the periodic table.
• metamorphic rock — Rock that forms when heat and/or pressure change an existing rock's structure or minerals without melting it.
• metamorphism — The process by which heat and/or pressure change an existing rock into metamorphic rock without melting it.
• meteorologist — A scientist who studies the atmosphere and uses weather data to make forecasts.
• micrometer — A thousandth of a millimetre, written µm — the working unit for cells, which run from about 1 µm for a bacterium to 50 µm for a plant cell.
• microorganism — Any organism too small to see unaided; a category of size, not of harm, and most are harmless or useful.
• mineral — A naturally occurring, inorganic solid with a definite chemical composition and an orderly, repeating crystal structure.
• mitigation — Reducing causes or drivers of a problem, such as greenhouse-gas emissions in a climate response.
• mitochondrion — The organelle, about a micrometre long, that transfers energy from food into a usable form during aerobic respiration.
• mitosis — The division that separates sister chromatids so each daughter cell receives a complete identical set; it produces no variation.
• mixture — Substances combined in variable proportions without one new fixed composition.
• model — A representation used to explain or predict selected features and relationships.
• molality — Moles solute per kilogram of solvent.
• molar mass — The mass of one mole of a substance in grams, numerically equal to the sum of the atomic masses in its formula (g/mol).
• molar volume — The volume occupied by exactly one mole of a gas; it is 22.4 L at STP for any ideal gas.
• molarity (M) — Concentration expressed as the number of moles of solute per liter of solution (M = mol ÷ L).
• mole — The SI unit of amount of substance, containing exactly 6.02214076×10²³ specified entities per mole.
• mole (mol) — The chemist's counting unit: one mole is exactly 6.022 × 10²³ particles of a substance.
• mole (n) — A count of 6.02 × 10²³ particles; the amount of gas, symbolized n, in the ideal gas law.
• mole ratio — A ratio between the amounts of any two substances in a reaction, taken directly from the coefficients of the balanced equation.
• molecular evidence — Similarities in DNA, RNA, and protein sequences among species; the more recently two species shared an ancestor, the more alike their molecules tend to be.
• molecular formula — Actual atom counts in a molecule.
• molecular geometry — The three-dimensional shape formed by the atoms in a molecule.
• molecule — A neutral group of atoms held together by covalent bonds.
• momentum — The 'quantity of motion' of an object, p = mv; a vector measured in kilogram-metres per second (kg·m/s).
• monomer — A small molecule that serves as a building block for a larger polymer.
• mRNA (messenger RNA) — A single-stranded working copy of a gene that carries its instructions out of the nucleus to a ribosome.
• multicellular — Made of many cells that divide the work between them, organised into tissues and in some organisms into organs.
• mutation — A change in the DNA sequence, arising from copying errors or damage; it occurs independently of whether it would be useful.
• mutualism — A relationship in which both participants gain, such as bees and flowering plants, or root fungi trading minerals for sugars.

N

• natural resource — Any material that comes from Earth and that people use, such as water, minerals, soil, or energy sources.
• natural selection — The process by which variants leaving more surviving offspring become more common; it needs heritable variation, differential reproduction and inheritance, and nothing else.
• negative feedback — A response that opposes the deviation that triggered it — too hot leads to cooling — which is the only arrangement that produces stability.
• net change — The difference between two opposing processes, which is all a sealed-chamber gas reading can report.
• net charge — The imbalance between positive and negative charge in the chosen system.
• net force — The overall force on an object after combining all individual forces, accounting for direction.
• net force (ΣF) — The single overall force found by adding every force on an object as a vector, accounting for direction.
• net primary productivity (NPP) — The energy producers have left after their own cellular respiration (NPP = GPP - respiration); it is the energy actually available to the rest of the food web.
• neuron — A cell specialised for carrying electrical signals, some running nearly a metre, with chemical junctions between them where signals can be modified.
• neutralization — A reaction between an acid and a base that produces a more neutral solution (plus a salt and water).
• neutron — A particle with no charge (neutral) found in the nucleus, alongside the protons.
• new moon — The phase when the Moon's unlit side faces Earth, making it nearly invisible in the sky; the start (and end) of the monthly cycle.
• newton (N) — The unit used to measure force; 1 newton = 1 kg·m/s².
• Newton's first law — An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by an unbalanced force.
• Newton's law of universal gravitation — Every pair of masses attracts with a force F = Gm₁m₂/r², where r is the distance between their centers.
• Newton's second law — Force equals mass times acceleration: F = ma.
• Newton's third law — For every action force, there is an equal and opposite reaction force.
• nitrification — Microbial oxidation pathways converting ammonia/ammonium through nitrite to nitrate under suitable conditions.
• nitrogen fixation — The conversion of unusable nitrogen gas (N₂) into ammonia or nitrates that plants can absorb, carried out mainly by certain bacteria (and by lightning).
• noble gases — The Group 18 elements with full valence shells, which makes them extremely unreactive.
• node — A branching point representing a common ancestor; two lineages sharing a recent node are more closely related than either is to one branching earlier.
• nondisjunction — Failure of the relevant chromosomes or chromatids to separate normally.
• nonmetal — An element that is typically dull, brittle, and a poor conductor; nonmetals cluster in the upper right.
• nonpoint-source pollution — Pollution that comes from many scattered, hard-to-pinpoint sources, such as fertilizer and oil washing off farms, lawns, and roads across an entire watershed.
• nonpolar bond — A covalent bond in which electrons are shared equally, with no separation of charge.
• nonrenewable resource — A natural resource that takes millions of years to form and is used up far faster than nature can replace it, such as fossil fuels and mined metals.
• normal — A line perpendicular to a surface at the point being considered.
• normal force — Contact support force perpendicular to a surface.
• normal force (N) — The support force a surface exerts perpendicular to itself; on level ground it equals the object's weight, but on an incline it is only mg cosθ.
• nuclear fission — The splitting of a heavy nucleus into lighter nuclei plus neutrons, releasing energy.
• nuclear fusion — The joining of light nuclei into a heavier nucleus, releasing even more energy; it powers the Sun.
• nucleic acid — A macromolecule (DNA or RNA) that stores and carries genetic information; its monomer is a nucleotide.
• nucleotide — The building block of DNA, made of a sugar, a phosphate group, and one of four nitrogen bases (A, T, C, or G).
• nucleus — The compartment holding almost all of a eukaryotic cell's DNA; it stores and issues instructions rather than building anything.
• nutrient — A substance a plant takes up through its roots, released into the soil largely by decomposers breaking down dead material.

O

• observation — A recorded detection or measurement under a stated method.
• octet rule — The tendency of atoms to gain, lose, or share electrons until their outermost shell holds a stable set of eight valence electrons.
• Ohm's law — The relationship V = IR linking voltage, current, and resistance for a resistor at constant temperature.
• operational definition — Stated rule for measuring or judging a property.
• orbit — A curved path arising from motion and gravitational interaction around a system.
• orbital — A region within a sublevel that can hold a maximum of two electrons.
• orbital period — Time required for one complete orbit.
• ore — A rock or mineral deposit that contains enough of a valuable metal or mineral to be worth mining and extracting.
• organ — Different tissues combined to do something none could alone; a stomach needs epithelial, muscle, nervous and connective tissue together.
• organ system — Organs working together on one broad function; an organ serving two functions, as the pancreas does, belongs to two systems.
• organelle — A structure inside a cell that does a particular job; the word means "little organ".
• organism — A living thing made of one or more cells that uses energy, responds, grows, reproduces and maintains stable internal conditions — characteristics applied as a set.
• osmosis — The diffusion of water across a selectively permeable membrane, from the weaker solution towards the stronger one.
• outcome — The specific thing measured in an evaluation; a result about runoff volume supports a claim about runoff volume.
• output force — The force the machine applies to a load.
• overproduction — The tendency of populations to produce far more offspring than the environment can support, which guarantees competition for limited resources.
• ovule — The structure inside the ovary holding an egg cell; after fertilisation it develops into a seed.
• oxidation — Increase in oxidation number; electron loss in a half-reaction.
• oxidative phosphorylation — ATP production coupled to electron transport and a proton gradient.
• oxidizing agent — Species that gains electrons and is reduced.

P

• P wave — A compression/expansion seismic wave that can propagate through suitable solids and liquids.
• paleomagnetism — Magnetic patterns frozen into ocean-floor rock as it cooled, recording the direction of Earth's magnetic field at the time — used as evidence for seafloor spreading.
• Pangaea — The single supercontinent that Wegener proposed all of today's continents were once joined together as, before it broke apart.
• parallel branch — A separate conducting route between shared connection points in a circuit.
• parallel circuit — A circuit with multiple paths, where each device can work independently of the others.
• parasitism — One organism gaining at another's expense over time without usually killing it quickly.
• parent isotope — The radioactive isotope that decays.
• particle factor i — Effective number of dissolved particles per solute formula unit in a model.
• particle model — A simplified explanation of matter using tiny moving constituents and their arrangement.
• passive transport — Movement of substances across the membrane down their concentration gradient, requiring no energy from the cell.
• pathogen — A microorganism or virus that causes disease; whether something acts as one often depends on where it is, as with skin bacteria in a wound.
• pattern — A relationship observed in records that may guide a test or explanation.
• Pauli exclusion principle — No orbital can hold more than two electrons, and those two must have opposite spins.
• peak force — Largest force recorded during a specified interval.
• per-capita consumption — The average amount of a resource used by one person; total demand depends on population size and per-capita consumption together.
• percent abundance — The percentage of an element's atoms found in nature that are a particular isotope.
• percent composition — Elemental mass divided by total mass, expressed as a percent.
• percent yield — The actual yield expressed as a percentage of the theoretical yield: (actual ÷ theoretical) × 100%.
• period — A horizontal row ordered by atomic number; in the introductory shell model, the period relates to the highest occupied principal shell.
• period (T) — The time it takes for one full wave to pass, T = 1 ÷ f.
• periodic table — A chart that organizes all known elements by increasing atomic number and by shared properties.
• periodic trend — A predictable pattern in a property that repeats as you move across periods or down groups.
• permanent deformation — Shape change remaining after the interaction.
• permeability — How readily water passes through connected pores.
• pH — A logarithmic measure of hydrogen-ion activity, approximated by concentration in dilute solutions. One unit corresponds to a tenfold difference; 0–14 is a common classroom range, not an absolute limit.
• pH scale — A logarithmic measure related to hydrogen-ion activity in water. Common dilute solutions near room temperature lie roughly within 0–14 with neutral near 7; this is not a universal range.
• phase — A physical state or distinct material region, such as solid or liquid.
• phase change — A physical change in which a substance moves between the solid, liquid, and gas states without becoming a new substance.
• phase diagram — A pressure-versus-temperature graph showing which phase is stable under each set of conditions, including the triple point and critical point.
• phenotype — The observable characteristic, which usually depends on many genes together with the environment rather than on one allele.
• phospholipid — A molecule with a phosphate 'head' and two fatty-acid 'tails' that is the main building block of cell membranes.
• phospholipid bilayer — The double layer of phospholipids — heads facing the water outside and inside, tails tucked together in the middle — that forms the membrane.
• photic zone — The sunlit upper layer of a body of water where there is enough light for photosynthesis.
• photon — A quantum of electromagnetic energy.
• photosynthesis — The reaction in chloroplasts that uses light energy to build glucose from carbon dioxide and water, releasing oxygen as a by-product.
• photovoltaic (PV) cell — A device, usually made of silicon, that converts sunlight directly into electricity.
• phylogenetic tree — A diagram of branching order and relationship; without a stated scale, branch lengths say nothing about timing.
• physical change — A change in form or appearance where no new substance is created.
• physical property — A characteristic measured or observed without changing chemical identity.
• pitch — How high or low a sound seems, determined by its frequency.
• pKa — Negative base-ten logarithm of Ka.
• Planck's constant (h) — The fundamental constant linking a photon's energy to its frequency, h ≈ 6.63 × 10⁻³⁴ J·s.
• plate boundary — A region where tectonic plates interact through relative motion.
• plateau — Approximately level portion of a graph.
• ploidy — Number of chromosome sets, distinct from DNA-copy amount.
• point-source pollution — Pollution that enters the environment from a single, identifiable location, such as a factory's discharge pipe or a power plant's smokestack.
• polar bond — A covalent bond in which electrons are shared unequally, creating a slightly positive end and a slightly negative end.
• polar molecule — A molecule with a slightly positive end and a slightly negative end because its atoms share electrons unequally.
• polarity — An uneven sharing of electrons that leaves a molecule with a positive end and a negative end (a dipole).
• polarizability — How easily a molecule's electron cloud can be distorted; larger, electron-rich clouds are more polarizable and give stronger dispersion forces.
• polarization — A slight shift or separation of positive and negative charge within material; the whole object can remain electrically neutral.
• pole — A north or south magnetic orientation region in the simple magnet model.
• pollination — The transfer of pollen to a stigma — delivery only, and separate from the fertilisation that may or may not follow.
• pollutant — A substance released into the environment, usually by human activity, at a level high enough to harm organisms or ecosystems.
• polyatomic ion — A charged group of covalently bonded atoms that acts as a single unit, such as nitrate (NO₃⁻) or sulfate (SO₄²⁻).
• polygenic trait — A trait influenced by variation at multiple genes.
• polymer — A large molecule made of many monomers bonded together in a chain.
• population — All the individuals of one species in a defined area at a defined time; both the boundary and the date are part of the definition.
• pore space — The spaces between soil particles, shared by water and air; its size determines drainage, water holding and oxygen supply.
• porosity — The fraction of a material’s volume that is pore space.
• position — Location relative to a chosen origin.
• position-time graph — A graph of position versus time; its slope at any point is the instantaneous velocity.
• positive feedback — A loop in which the response amplifies the change, driving a process to completion before it shuts off.
• potential energy — Energy associated with relative position or configuration in an interacting system.
• power — Rate of energy transfer or work.
• precipitate — A solid that forms and settles out when two solutions react with each other.
• precipitation — Water falling back to Earth's surface as rain, snow, sleet, or hail once droplets or ice crystals grow heavy enough.
• predation — One organism killing and eating another; its effects often travel beyond the prey to species the predator never touches.
• pressure — Force acting per unit area.
• prevailing westerlies — The belt of winds in the mid-latitudes, including New York, that generally blows from west to east, steering most weather systems that way.
• probability — The proportion of an outcome expected over many offspring — not a guarantee about any small number of them.
• producer — An organism that builds organic matter using an energy source rather than feeding on organic food.
• product — A substance made by a reaction; the oxygen you breathe is a product of photosynthesis.
• projectile — An object moving through the air acted on only by gravity after its launch (air resistance neglected).
• projection — A conditional estimate based on stated future assumptions.
• prokaryote — A member of Bacteria or Archaea whose cell lacks a membrane-bound nucleus. Its DNA is organized in a nucleoid; ribosomes and membranes support coordinated functions.
• prokaryotic — Having DNA loose in the cytoplasm with no enclosing nucleus, as bacteria do; typically 1 to 5 micrometres long.
• proper time — Time measured by a clock present at both events.
• protein — A macromolecule made of amino acids that does most of the cell's work; its monomer is an amino acid.
• proton — A positively charged particle found in the nucleus.
• prototype — A first version built to be tested rather than to be final, such as one planted section instead of a whole site.
• Punnett square — A probability model of gamete combinations under stated assumptions.
• pure substance — Matter with a characteristic chemical composition.

Q

• quadrat — A frame of known area placed on the ground so everything inside can be counted, giving a density that can be scaled up.
• qualification — A statement of what a claim does not cover — conditions, period, population — which narrows a claim to what the evidence carries.

R

• radiation — Energy transfer by electromagnetic waves, which can cross empty space.
• radioactivity — The spontaneous emission of particles or energy from an unstable atomic nucleus.
• range — The distance from the lowest value to the highest, which shows how consistent a set of measurements was.
• rarefaction — A region of a longitudinal wave where particles are spread farther apart.
• reabsorption — Taking useful substances back from the filtrate into the blood, often by active transport against a gradient, which is why the kidney is so energy-hungry.
• reactant — A substance consumed by a reaction. Light is not one, because it supplies energy rather than matter.
• reaction quotient Q — The equilibrium-expression ratio evaluated for current conditions.
• reaction rate — How fast reactants are used up or products form, often measured as a change in concentration or amount per unit of time.
• real vs. virtual image — A real image forms where light rays actually converge and can be projected on a screen; a virtual image only appears to come from where rays seem to originate.
• reasoning — The connection between evidence and claim, usually a mechanism explaining how one would produce the other; restating the evidence is not reasoning.
• receiver — Object on which a named force acts.
• receptor — A structure that detects one particular kind of stimulus; specificity is structural, which is why an eye cannot hear.
• recessive — Not expressed in the heterozygote’s modeled phenotype under complete dominance.
• recharge — Water entering groundwater storage.
• reclamation — Restoring mined or degraded land to a stable, safer, and useful condition after resources have been extracted.
• recombination — Exchange of corresponding segments between paired chromosomes during meiosis, so a gamete's chromosomes are mixtures rather than whole parental ones.
• redshift — A shift of identified spectral features toward longer wavelengths.
• reduction — Decrease in oxidation number; electron gain.
• reference level — The chosen height where gravitational potential energy is defined as zero; only changes in PE have physical meaning.
• reflection — Light being redirected from a surface.
• reflex — A fast involuntary response using a short route through the spinal cord, with the brain informed rather than consulted.
• refraction — A change in light propagation speed at a medium boundary, commonly with oblique bending.
• regulatory sequence — DNA that helps control how a gene is expressed.
• relative age — Order without numbers: in undisturbed layers, deeper is older, which gives sequence but no durations.
• relative dating — Determining whether a rock layer or fossil is older or younger than another — not its exact age in years.
• relative humidity — A ratio comparing actual vapor conditions with the saturation reference at the current conditions, normally expressed as a percent.
• renewable energy — Energy drawn from sources that nature replenishes on a human timescale — sunlight, wind, flowing water, Earth's internal heat, and plant matter.
• renewable resource — A natural resource that is naturally replenished on a human timescale, such as sunlight, wind, flowing water, and timber if harvested responsibly.
• repel — Push apart.
• replicate — An independent repetition of an experimental condition.
• replication — Copying the DNA, with each strand acting as a template for its partner, so that each chromosome becomes two identical joined copies.
• representative particle — The basic unit being counted — an atom for an element, a molecule for a molecular compound, or a formula unit for an ionic compound.
• reproduction — The production of new individuals; success is measured in offspring that survive to reproduce, not offspring produced.
• reproductive isolation — Any barrier — geographic, behavioral, temporal, or biological — that prevents two populations from interbreeding, allowing them to evolve independently.
• reservoir — A store of matter — atmosphere, ocean, organisms, soil, rock — characterised by how much it holds and how fast matter enters and leaves.
• residual — Difference between an observed value and a fitted prediction.
• resilience — The ability to absorb a disturbance and return to a similar state; it comes from functional overlap and is specific to what is being resisted.
• resistance — Anything that opposes the flow of current, measured in ohms (Ω).
• resistivity — A property of a material itself that sets how strongly it resists current; it, along with a wire's length and thickness, determines the wire's resistance.
• resolution — The smallest change a measuring instrument can distinguish under its method.
• resonance — Multiple electron drawings representing delocalized bonding in one structure.
• respiration — Cellular processes releasing usable energy from organic material, with associated matter changes.
• response — What the body finally does — a muscle contracting or a gland releasing something — at the end of the pathway.
• reversible reaction — A reaction that can proceed in both the forward and reverse directions, written with a double arrow (⇌).
• revision — A reasoned change to a model or explanation in response to evidence or an identified limit.
• revolution — A body’s orbital movement around another body or system.
• ribosome — A machine about 20 nanometres across that builds a protein by joining amino acids in the order an instruction specifies.
• ribozyme — An RNA molecule with catalytic activity; enzyme catalysis is not restricted to proteins.
• richness — Number of species detected under specified effort.
• rock cycle — The continuous, looping set of processes — cooling, weathering and erosion, lithification, metamorphism, and melting — that recycle Earth's materials among igneous, sedimentary, and metamorphic rock.
• Roman numeral — A notation specifying charge of a variable-charge metal cation.
• rotation — A body turning about its axis.
• runoff — Water flowing across the surface to streams and rivers instead of soaking in; how much depends on the surface.

S

• S wave — A seismic shear wave; liquids do not sustain its restoring shear behavior.
• salinity — The concentration of dissolved salts in water.
• salt — An ionic compound formed from the positive ion of a base and the negative ion of an acid during neutralization — table salt (NaCl) is just one example.
• sample — The part of an area actually counted, from which the whole is estimated; everything depends on whether it represents the whole.
• sampling bias — Any systematic difference between the sample and the whole, usually introduced by choosing where to sample while looking at the area.
• saturated hydrocarbon — A hydrocarbon framework with only single carbon-carbon bonds.
• saturated solution — A solution that holds all the solute it can at a given temperature; any extra solute added will simply not dissolve.
• scalar — A quantity with magnitude only.
• scale — A stated ratio connecting represented and actual quantities with units.
• scale bar — A line drawn on an image with its real length stated, so a size can be measured from the image however it was later enlarged.
• scenario — A specified set of assumptions about future drivers.
• seafloor spreading — The process by which new ocean floor forms at mid-ocean ridges and spreads outward in both directions, providing the mechanism for continental drift.
• sediment — Small, loose pieces of rock, mineral, or organic material produced by weathering.
• sedimentary rock — Rock that forms when weathered sediment is deposited in layers, then compacted and cemented together.
• seismic wave — Energy released by an earthquake that travels through Earth as vibrations, including P-waves, S-waves, and surface waves.
• selection pressure — The force determining which individuals leave more offspring; applied hard and long it produces a population excellent at one thing and uniform in everything else.
• selective breeding — Repeatedly breeding from individuals with a desired characteristic; it acts on existing variation and cannot create a characteristic nothing shows.
• selective permeability — The property of letting some substances cross the membrane while blocking or controlling others; also called semipermeability.
• selective pressure — An environmental factor — such as predation, climate, disease, or scarce food — that causes some variants in a population to survive and reproduce more than others.
• selectively permeable — Letting some substances cross while holding others back, which is what turns simple diffusion into something a cell can live on.
• semiconservative — Each copied duplex retains an original strand and gains a new complementary strand in the standard model.
• semiconservative replication — The way DNA copies itself so that each new double helix keeps one original strand and one newly built strand.
• semimajor axis — Half the longest diameter of an ellipse.
• sensitivity analysis — Recalculation using plausible changes in uncertain inputs.
• series circuit — A circuit with a single path, where a break anywhere stops the current everywhere.
• set point — The ideal value a feedback system works to maintain, like the target temperature on a thermostat.
• sexual reproduction — Production of offspring by the fusion of gametes from two parents, giving new combinations and therefore variation every generation.
• shared derived trait — A characteristic that first evolved in a common ancestor and is shared by all of its descendants, used to define groups (clades) on a cladogram — such as a backbone or hair.
• shielding (screening) — The way inner-shell electrons block, or screen, valence electrons from feeling the full charge of the nucleus.
• signal — Changing physical pattern used to carry information.
• significant figures — Digits used to communicate the precision of a measurement.
• simple machine — A device that changes the size or direction of an applied force.
• simple payback — Upfront cost divided by annual savings in a simplified model.
• single-replacement reaction — A reaction where one element replaces another element in a compound (A + BC → AC + B).
• sister chromatids — The two identical copies of a chromosome joined after replication; counted as one chromosome until they separate.
• sister lineages — Two lineages joined at their most recent shared node.
• skeletal muscle — Muscle attached to bones that shortens when stimulated, generating a pull; it cannot push, which is why muscles work in opposing pairs.
• slope — Change in vertical value divided by change in horizontal value.
• small intestine — The organ where most chemical digestion is completed and most absorption happens, with enzymes delivered into it by the pancreas and bile by the liver.
• soil — A mixture of mineral particles, organic matter, water and air; the air in the pores is as essential as the water.
• soil degradation — The decline in soil quality caused by erosion, loss of nutrients, salinization, compaction, or contamination.
• soil horizon — A distinct layer of soil — such as the O, A, B, and C horizons — that differs from the layers above and below it in color, texture, and composition.
• solid — A state of matter with a fixed shape and volume; particles vibrate in place, tightly packed.
• solstice — One of two days a year (around June 21 and December 21) when a hemisphere is angled most directly toward or away from the Sun, giving the year's longest or shortest day.
• solubility — The maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature, often given in grams per 100 g of water.
• solute — The substance dissolved in a solution.
• solution — A mixture with dissolved particles distributed through a solvent at the particle scale.
• solvent — The substance that does the dissolving in a solution, usually present in the larger amount; water is the most common one.
• source — Object initiating the sound disturbance.
• specialization — Being built for one job, which buys efficiency and costs independence — a muscle cell contracts superbly and cannot feed itself.
• specialized cell — A cell built for one job, such as a root hair, guard cell or red blood cell, which no general diagram represents.
• speciation — The evolutionary process by which one ancestral species splits into two or more distinct species that can no longer interbreed.
• species diversity — A measure combining species richness (how many species) with evenness (how equally abundant they are).
• species richness — The number of different species recorded under a stated method.
• specific heat (c) — The energy needed to raise the temperature of 1 gram of a substance by 1°C; for liquid water it is 4.18 J/g°C.
• spectator ion — An ion unchanged by the represented reaction.
• spectrum — A range of light components distinguished by wavelength.
• speed — How fast an object moves, found by dividing distance traveled by the time it took.
• speed of light (c) — The constant speed of all EM waves in a vacuum, about 3 × 10⁸ m/s — the universe's ultimate speed limit.
• spontaneous — Thermodynamically favored under stated conditions; not necessarily rapid.
• spontaneous generation — The discredited belief that living things could arise directly from non-living matter, overturned by evidence that cells only come from cells.
• spring constant — Stiffness parameter in a linear elastic model.
• spring constant (k) — A measure of a spring's stiffness, in newtons per metre (N/m); it appears in Hooke's law F = kx.
• spring tide & neap tide — Spring tides have a larger typical tidal range near new/full phases when lunar and solar tidal axes align; neap tides have a smaller range near quarter phases when those axes are roughly perpendicular. Local coast/basin dynamics modify the actual tides.
• stakeholder — Person or group affected by the design.
• standing biomass — The biological material present at one instant, distinct from production per time.
• standing wave — A stationary wave pattern of fixed nodes and antinodes formed when a wave reflects and interferes with itself, as on a guitar string.
• star — A body that emits light through its internal energy processes, such as the Sun.
• state function — A property determined by state rather than path.
• static friction — The friction force between two surfaces that are not yet sliding; it adjusts up to a maximum to prevent motion.
• station model — A small cluster of symbols and numbers on a weather map that packs in one location's temperature, wind, and cloud cover at a glance.
• stewardship — Using and managing natural resources responsibly so that future generations can still meet their own needs.
• stimulus — A change the body can detect — heat, pressure, light, a chemical — which exists in the world whether or not an organism has a receptor for it.
• stoichiometry — The use of a balanced equation to calculate the amounts of reactants and products in a reaction.
• stop codon — An mRNA codon such as UAA that signals termination instead of adding another amino acid.
• storage change — Inputs minus outputs across a stated boundary; a store can grow or shrink while matter is conserved overall.
• STP — Standard Temperature and Pressure: 0°C (273 K) and 1 atm, the reference conditions for comparing gases.
• stratification — Layering of water with different densities.
• streak — The color of a mineral’s powder under a defined observation method.
• strong acid — An acid that ionizes completely in water, releasing nearly all of its hydrogen ions.
• structural isomers — Compounds with the same molecular formula but different connectivity.
• subduction — One lithospheric plate sinking beneath another at a suitable convergent boundary.
• sublevel — A subdivision of an energy level, labeled s, p, d, or f, each with a characteristic shape and capacity.
• sublimation — The phase change directly from solid to gas, skipping the liquid state; deposition is the reverse.
• subscript — The small number written after an element's symbol, showing how many atoms of it are in one molecule.
• substrate — The specific molecule (or molecules) an enzyme acts on.
• succession — Changes in community composition over time.
• superposition — The law stating that in an undisturbed sequence of rock layers, the oldest layer is on the bottom and the youngest layer is on top.
• surface area — How much surface is exposed; it governs the rate of any surface process, which is why food is chewed and the gut lining is folded.
• surface tension — An effect of particle interactions at a liquid surface that can influence small surface objects.
• survivorship curve — A graph of how many individuals of a cohort survive over their lifespan, classed as Type I, II, or III.
• sustainability — Meeting the needs of people today without ruining the ability of future generations to meet their own needs.
• symbiosis — A close, long-term relationship between two different species, which may benefit both (mutualism), benefit one without affecting the other (commensalism), or benefit one at the other's expense (parasitism).
• synodic month — Average new-to-new lunar phase cycle of about 29.5 days; distinct from the approximately 27.3-day orbit relative to distant stars.
• synthesis reaction — A reaction that combines smaller molecules into a larger one — a building-up reaction.
• synthetic material — A material made through human-directed processing, often involving chemical reactions.
• system boundary — The chosen limit specifying what is included and which exchanges cross it.
• systematic error — A recurring offset caused by a method or instrument.

T

• target cell — A cell carrying the receptor for a particular hormone, which is how a message broadcast to the whole bloodstream is acted on only where it matters.
• taxonomy — The science of naming, describing, and classifying organisms into a nested hierarchy of groups: domain, kingdom, phylum, class, order, family, genus, and species.
• temperature — A property related to average microscopic particle motion in a simple material model.
• ten percent rule — The generalization that only about 10% of the energy stored at one trophic level is passed on to the next; roughly 90% is used or lost as heat.
• tendon — A dense, barely stretchy cord of collagen attaching muscle to bone, so that a contraction is transmitted rather than absorbed.
• tension — The pulling force carried along a rope, string, or cable, directed along its length.
• terrestrial planet — One of the four small, dense, rocky inner planets — Mercury, Venus, Earth, or Mars.
• testable question — A question naming what will be changed, what will be measured and in what units, and what is held the same — so that a result could contradict it.
• theoretical yield — The maximum amount of product predicted by stoichiometry, assuming the reaction goes perfectly and the limiting reactant is fully converted.
• thermal energy — Internal energy associated with random particle motion and particle interactions; temperature alone does not give its total amount.
• thermal equilibrium — A state in which interacting objects have the same temperature.
• thermoplastic — A polymer material that can soften with heating under appropriate conditions.
• threshold — Decision boundary for assigning a measurement to a category or symbol.
• threshold frequency — Minimum photon frequency for single-photon photoemission.
• tidal range — The difference between high and low water levels using the same reference.
• tide — The regular rise and fall of ocean water levels, caused mainly by the Moon's gravity, with a smaller contribution from the Sun.
• tilt — The 23.5° lean of Earth's axis away from vertical, relative to its orbital plane.
• tissue — Cells of similar type working together on one job — muscle contracts, epithelium covers and controls surfaces, nervous tissue signals, connective tissue supports.
• titration — A lab procedure that slowly adds a solution of known concentration to another until the reaction is exactly complete, used to find an unknown concentration.
• tonicity — A solution’s effect on cell volume through net water movement, depending on effectively nonpenetrating solutes, membrane permeability and the stated conditions.
• torque — Turning effect of a force about a specified pivot.
• total internal reflection — When light in a higher-index medium strikes a boundary beyond the critical angle and is entirely reflected back, as in a fiber-optic cable.
• trace fossil — A footprint, burrow, tooth mark or dropping, preserving behaviour rather than any part of the organism.
• trade-off — A benefit gained at the cost of another desirable feature.
• tradeoff — A situation where improving one outcome can limit or worsen another valued outcome.
• trajectory — The path a projectile follows, which is a parabola.
• transcription — The process of copying a gene's DNA into a strand of messenger RNA (mRNA).
• transect — A line across an area with samples taken at intervals, used when the question is about change along a gradient rather than an average.
• transfer efficiency — The fraction of energy at one position reaching the next, expressed as a percentage; measured values range from about one per cent to over twenty.
• transform boundary — A place where two tectonic plates slide horizontally past each other without much crust being created or destroyed.
• transition metals — The d-block metals in the center of the table (groups 3–12), including iron, copper, and gold.
• transitional fossil — A fossil that shows a mix of traits from an ancestral group and its descendants, documenting change between forms — such as Tiktaalik, between fish and four-legged animals.
• translation — The process of reading mRNA codons at a ribosome to build a chain of amino acids.
• transmission — Whether a change can reach offspring, which depends on the cell: only changes in cells producing gametes can be passed on.
• transpiration — Water passing through plants and entering the atmosphere as vapor.
• transverse wave — A wave in which the particles (or fields) vibrate perpendicular to the direction the wave travels, like a wave on a rope or light.
• triple bond — A covalent bond in which three pairs of electrons (six electrons) are shared.
• trophic level — A feeding step in an ecosystem — producers, then primary consumers, then secondary consumers, and so on.
• trophic position — A place in the feeding sequence — producers, then those eating them, then those eating those.
• turbidity — A measure of water cloudiness associated with suspended material under a stated method.
• turgor — The outward pressure of a full vacuole against the cell wall, which holds a non-woody plant upright; losing it is wilting.

U

• unbalanced forces — Forces that combine to a net force that isn't zero, causing acceleration.
• uncertainty — A stated limit in measurement or knowledge, distinct from a careless mistake.
• unicellular — Made of a single cell that carries out every life process itself, as a Paramecium or a bacterium does.
• unit — A defined reference used to express a measurement.
• unknown — The substance whose amount you are solving for.
• urine — What remains of the filtrate after reabsorption: mostly water with urea and surplus salts, concentrated or dilute according to what the body needs.

V

• vacuole — A membrane-bound sac; in a plant cell one large central vacuole of water and dissolved substances can fill most of the volume.
• valence electron — An electron in an atom's outermost shell; it mostly determines how the atom bonds with others.
• valence electrons — Electrons relevant to the chemical bonding model. Main-group outer-shell patterns are a useful case; transition elements can involve nearby d states.
• variation — Differences among observations that a single average may not describe.
• vector — A quantity with magnitude and direction.
• vein — A vessel carrying blood back towards the heart, wider and thinner-walled, with valves preventing backflow at low pressure.
• velocity — Speed in a specific direction, like 60 km/h north.
• velocity-time graph — A graph of velocity versus time; its slope is the acceleration and the area beneath it is the displacement.
• ventilation — The mechanical movement of air in and out of the lungs as the diaphragm and ribs change the volume of the chest; nothing chemical happens yet.
• vertical component — The part of a projectile's motion along the direction of gravity; it accelerates downward at g.
• vestigial structure — A feature reduced from a larger ancestral form, such as a whale's hip bones; reduced is the claim, not useless.
• vibration — Repeated motion around a rest position.
• voltage — The electrical push (potential difference) that drives current, measured in volts (V).
• voltage (potential difference) — The electrical energy delivered to each unit of charge, V = W/Q, measured in volts (V).
• volume — The space occupied by an object.
• VSEPR theory — Valence Shell Electron Pair Repulsion — the idea that electron groups around a central atom spread out as far apart as possible, which sets the molecule's shape.
• vulnerability — How susceptible a population or structure is to damage.

W

• waning — A decrease in the Moon’s observed illuminated fraction over the phase sequence.
• water vapor — Water in the normally invisible gas phase.
• watershed — An area whose drainage connects toward a common outlet under a defined landscape model.
• watt — Unit of power: one joule per second.
• watt (W) — The SI unit of power; one watt is one joule per second (1 W = 1 J/s).
• wave — A traveling disturbance that transfers energy. In a simple mechanical wave, particles mainly move locally rather than travel the entire distance with the disturbance.
• wave-particle duality — The experimental fact that light (and matter) behaves as both a wave and a stream of particles, depending on how it is observed.
• wavelength (λ) — The distance between two matching points on a wave, like crest to crest.
• waxing — An increase in the Moon’s observed illuminated fraction over the phase sequence.
• weak acid — An acid that ionizes only partially in water, so most of its molecules stay intact at any moment.
• weather — Atmospheric conditions at a particular place over a relatively short interval.
• weathering — Breakdown or alteration of material in place.
• weight — The force of gravity on an object's mass, calculated as weight = mass × g (W = mg).
• weighted average — An average in which each value is multiplied by how common it is, so more abundant values count more.
• wetland — Land saturated with water at least part of the year (marshes, swamps, bogs); among the most productive ecosystems, it filters water and buffers floods.
• white dwarf — A compact stellar remnant left by a lower-mass star after later stages.
• work — Energy transferred by a force acting through a displacement.
• work function — Minimum energy needed to eject an electron from a surface in the model.
• work-energy theorem — The net work done on an object equals its change in kinetic energy: W_net = ΔKE.

Z

• zygote — The single cell formed at fertilisation, carrying a combination of both parents' genetic information; not a miniature organism but the cell one is built from.

Browse all courses