Revision reference
GCSE & A-Level revision glossary
What is the StudyVector revision glossary?
StudyVector's revision glossary defines 55 GCSE and A-Level terms in 40–80 word answers — board-aligned to AQA, Edexcel and OCR. Each entry pins a direct definition, shows where the term appears in exams, and links into the matching practice hub. Independent reference, free to read, no exam-board affiliation.
Browse by subject + level. Each term links to its full definition page with exam-board notes and practice routes.
Biology — A-Level
- Action potentialAn action potential is the rapid voltage change across a neurone membrane that propagates an electrical signal. Resting potential (~−70 mV) is maintained by the Na⁺/K⁺ pump. A stimulus opens voltage-gated sodium channels → depolarisation (up to ~+40 mV) → potassium channels open → repolarisation → temporary hyperpolarisation → return to resting potential. The all-or-nothing principle: action potentials only fire above a threshold (~−55 mV), and all action potentials in a given neurone have the same amplitude.
- Calvin cycleThe Calvin cycle is the second stage of photosynthesis, happening in the chloroplast stroma. It uses ATP and NADPH (from the light-dependent reactions) to fix carbon dioxide into glyceraldehyde-3-phosphate (G3P), eventually producing glucose. Three phases: CO₂ fixation (catalysed by RuBisCO), reduction (G3P from GP using ATP + NADPH), and regeneration of ribulose bisphosphate (RuBP). Named after Melvin Calvin (1961 Nobel Prize). Independent of light directly, but stops when ATP/NADPH run out.
- ChemiosmosisChemiosmosis is the synthesis of ATP driven by a proton (H⁺) gradient across a membrane. In photosynthesis, light energises electrons that pump H⁺ into the thylakoid lumen; the gradient drives ATP synthase. In respiration, the electron transport chain pumps H⁺ into the mitochondrial inter-membrane space; ATP synthase converts the flow-back into ATP. The principle (proposed by Peter Mitchell, 1978 Nobel Prize) is central to both energy-converting processes in cells.
- Enzyme kineticsEnzyme kinetics describes the rate of enzyme-catalysed reactions. Rate depends on substrate concentration, enzyme concentration, temperature and pH. As substrate concentration rises, rate increases until all active sites are saturated (Vmax). The Michaelis constant (Km) is the substrate concentration at half Vmax — a low Km means high enzyme affinity. A-Level Biology examines competitive inhibition (high [S] outcompetes inhibitor) and non-competitive inhibition (binds away from active site; Vmax decreases).
- Hardy-Weinberg principleThe Hardy-Weinberg principle states that allele and genotype frequencies in a population remain constant generation-to-generation in the absence of evolutionary forces. Five assumptions: no mutation, random mating, no gene flow, infinite population size, no natural selection. Useful as a null hypothesis. Equations: p + q = 1 (allele frequencies); p² + 2pq + q² = 1 (genotype frequencies). A-Level Biology uses it to predict allele/genotype frequencies given carrier rates.
- Photosynthesis (light-dependent reactions)The light-dependent reactions of photosynthesis happen in the thylakoid membranes of chloroplasts. Light excites electrons in chlorophyll, driving an electron transport chain that pumps H⁺ into the thylakoid lumen. The H⁺ gradient drives ATP synthase (photophosphorylation). Water is split (photolysis) to replace lost electrons, producing O₂. NADP is reduced to NADPH. ATP and NADPH then power the Calvin cycle in the stroma.
Biology — Both
- DNA replicationDNA replication is the process by which a cell copies its DNA before division. The double helix unwinds at a replication fork. Each strand serves as a template; DNA polymerase adds complementary nucleotides (A-T, G-C). The leading strand is synthesised continuously; the lagging strand is built in Okazaki fragments. The result is two identical double helices, each with one original (parent) strand and one new (daughter) strand — hence 'semi-conservative'. Demonstrated by Meselson and Stahl in 1958.
- Mitosis vs meiosisMitosis produces two genetically identical diploid daughter cells from one parent cell — used for growth, repair and asexual reproduction. Meiosis produces four genetically different haploid gametes from one parent diploid cell — used for sexual reproduction. The genetic variation in meiosis comes from independent assortment (random chromosome alignment in metaphase I) and crossing over (chromatid exchange in prophase I). A-Level Biology examines the stages of both in detail.
- Natural selectionNatural selection is the mechanism Darwin identified for evolution: in any population there is genetic variation; individuals with advantageous traits are more likely to survive and reproduce; their advantageous alleles increase in frequency over generations. The four requirements: variation, heritability, differential survival/reproduction, and time. GCSE and A-Level Biology examine examples including peppered moth industrial melanism, antibiotic resistance in bacteria, and finch beak diversity in the Galápagos.
Chemistry — Both
- Atom economyAtom economy measures the proportion of reactant atoms that end up in the desired product: (Mr of desired product / sum of Mr of all products) × 100%. Unlike percentage yield (which accounts for reaction efficiency), atom economy is a theoretical maximum based on stoichiometry. High atom-economy reactions are greener — fewer waste atoms. Addition reactions typically have 100% atom economy; substitution reactions produce by-products and have lower atom economy.
- Avogadro's numberAvogadro's number is the number of particles in one mole of any substance: 6.022 × 10²³ mol⁻¹ (named after the chemist Amedeo Avogadro). One mole of any element or compound contains exactly this many atoms, molecules or formula units. GCSE Chemistry uses it for mole-particle calculations; A-Level extends to lattice energy and molar enthalpy calculations.
- Half-equationsA half-equation shows just the oxidation or just the reduction step of a redox reaction, including the electrons transferred. Example: Zn → Zn²⁺ + 2e⁻ (oxidation half). Half-equations must balance for both atoms and charge. Combining the two half-equations (so electrons cancel) gives the overall ionic equation. Used heavily in A-Level electrochemistry, electrolysis and redox titrations.
- Ionic bondingIonic bonding is the electrostatic attraction between oppositely-charged ions, formed when atoms transfer electrons to achieve full outer shells. A metal donates electron(s) to become a positive cation; a non-metal accepts to become a negative anion. The resulting compound has high melting/boiling points, conducts electricity when molten or dissolved (free ions to carry charge), and is typically brittle. Examples: NaCl, MgO, CaCl₂.
- Percentage yieldPercentage yield compares the actual mass of product obtained from a reaction with the maximum theoretical mass calculated from stoichiometry: (actual mass / theoretical mass) × 100%. Real-world yields are usually below 100% due to incomplete reactions, side products, or losses during purification. A high percentage yield is a key indicator of an efficient industrial process.
- Redox reactionsA redox reaction involves simultaneous oxidation and reduction. Oxidation: loss of electrons (or gain of oxygen / loss of hydrogen). Reduction: gain of electrons (or loss of oxygen / gain of hydrogen). The mnemonic OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). Identify redox by changes in oxidation number. A-Level Chemistry extends to half-equations and redox titrations using KMnO₄ or I₂/Na₂S₂O₃.
- TitrationTitration is a quantitative analysis technique that determines the concentration of an unknown solution by reacting it with a standard solution of known concentration until a clear end point. For acid–base titrations, an indicator (phenolphthalein, methyl orange) changes colour at the end point; for redox titrations, a self-indicating species like potassium manganate(VII) does the job. Required practical on every UK board.
Chemistry — GCSE
Chemistry — A-Level
- Le Chatelier's principleLe Chatelier's principle states that when a dynamic equilibrium is disturbed, the system shifts to partially counteract the change. Increase the concentration of a reactant → equilibrium shifts toward products. Increase temperature → equilibrium shifts in the endothermic direction. Increase pressure → equilibrium shifts toward the side with fewer gas molecules. A-Level Chemistry uses it to predict yield changes in industrial processes (Haber, Contact).
- Organic mechanismAn organic mechanism shows the step-by-step movement of electron pairs during a reaction, drawn with curly arrows. Each arrow starts from a lone pair, double bond, or single bond and ends at the atom or bond receiving the electrons. Major mechanism types at A-Level: free-radical substitution (alkane + halogen + UV), electrophilic addition (alkenes), nucleophilic substitution (halogenoalkanes), elimination, electrophilic substitution (benzene), nucleophilic addition. Examiners reject arrows drawn the wrong way around.
- pH and bufferspH = −log₁₀[H⁺]. A buffer solution resists pH change when small amounts of acid or base are added; it contains roughly equal concentrations of a weak acid and its conjugate base (or a weak base and its conjugate acid). Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). A-Level Chemistry uses buffers in biological-relevant contexts (blood pH ~7.4 buffered by HCO₃⁻/H₂CO₃) and industrial processes.
Exam admin — Both
- JCQJCQ (Joint Council for Qualifications) is the membership organisation for the UK's eight largest awarding bodies — AQA, Pearson Edexcel, OCR, WJEC, CCEA, City & Guilds, OCR Cambridge Nationals and others. JCQ publishes the rules for how exams are conducted in schools and colleges: access arrangements, malpractice, AI use in non-exam assessment, and exam-day procedures. JCQ does not set or mark papers.
- OfqualOfqual is the Office of Qualifications and Examinations Regulation — the non-ministerial UK government department that regulates qualifications, exams and assessments in England (with parallel bodies in the devolved nations). Ofqual approves GCSEs, A-Levels and vocational qualifications and oversees how exam boards (AQA, Pearson Edexcel, OCR, WJEC) set and mark papers. It does not write exams itself.
Exam admin — GCSE
Exam admin — A-Level
Maths — A-Level
- Binomial expansionBinomial expansion expands (a + b)^n using (a + b)^n = Σ (n choose k) a^(n−k) b^k for positive integer n. For rational/negative n the expansion is an infinite series, valid only for |b/a| < 1. A-Level Maths examines both forms; Further Maths extends to approximations. Sloppy work with the validity condition is a high-frequency lost mark.
- Chain ruleThe chain rule differentiates composite functions: if y = f(g(x)) then dy/dx = f'(g(x)) · g'(x). It appears across A-Level Maths and Further Maths whenever an outer function wraps an inner function — common examples are sin(2x), e^(3x²) and (1+x²)⁵. Without the chain rule, every composite differentiation drops marks; with it, the standard trick is to spot the inner function first.
- Differential equationsA differential equation relates a function to its derivatives. A-Level Maths examines first-order separable equations (dy/dx = f(x)·g(y)), solved by separating variables and integrating both sides. Applications include exponential growth/decay (dN/dt = kN), Newton's law of cooling and rate-of-reaction models. Further Maths extends to second-order equations and integrating factors.
- Implicit differentiationImplicit differentiation differentiates equations where y is not isolated, e.g. x² + y² = 25. Treat y as a function of x and apply the chain rule whenever you differentiate a y term — d(y²)/dx becomes 2y · dy/dx. Used heavily for circles, ellipses and tangent/normal problems where rearranging to y = f(x) is impractical.
- Integration by partsIntegration by parts is the integral analogue of the product rule: ∫ u dv/dx dx = uv − ∫ v du/dx dx. Use it whenever the integrand is a product of two functions where one becomes simpler under differentiation (typically polynomial × exponential, polynomial × trig, or x × log x). The LIATE rule helps pick u: Logarithm, Inverse trig, Algebraic, Trig, Exponential.
- P-valueA p-value is the probability of observing test statistics at least as extreme as the one obtained, assuming the null hypothesis is true. A small p-value (typically below 0.05) is evidence against the null hypothesis. A-Level Maths examines p-values for binomial hypothesis tests and normal hypothesis tests. A p-value alone is not a probability that the null is true — that's a common interpretation slip examiners penalise.
- Parametric equationsParametric equations express x and y separately as functions of a parameter t, e.g. x = cos(t), y = sin(t). They model curves that aren't conveniently expressed as y = f(x) — including circles, ellipses, and projectile paths. A-Level Maths uses the chain rule dy/dx = (dy/dt) / (dx/dt) to find gradients, and substitution to eliminate the parameter and recover the Cartesian equation.
- Vectors in 3DA-Level Maths extends vectors from 2D to 3D using the unit vectors i, j and k. Examinable skills: magnitude (|v| = √(x² + y² + z²)), addition/subtraction, scalar multiplication, the equation of a line r = a + λd, scalar (dot) product, and using the dot product to test perpendicularity (v · w = 0) or find angles between vectors.
Maths — GCSE
- Circle theoremsGCSE Maths circle theorems are a closed set of geometric rules about angles in a circle. The seven examinable rules: angle at the centre = 2× angle at the circumference; angle in a semicircle = 90°; angles in the same segment are equal; opposite angles in a cyclic quadrilateral sum to 180°; tangent meets radius at 90°; alternate segment theorem; tangents from an external point are equal.
- Completing the squareCompleting the square rewrites a quadratic ax² + bx + c into the form a(x + p)² + q. It exposes the minimum/maximum point of the parabola (at x = −p, y = q) and provides a route to the quadratic formula. GCSE Higher and A-Level both examine it — typical questions ask to express in completed-square form, find the turning point, or sketch the curve.
- Percentage changePercentage change is the difference between a new value and an original value, expressed as a percentage of the original: ((new − old) / old) × 100. GCSE Maths uses it for percentage increase/decrease, reverse percentages (finding the original before a known change), and multiplier methods (×1.05 for a 5% rise). The multiplier method is fastest and least error-prone for higher-tier work.
- Pythagoras' theoremPythagoras' theorem states that in a right-angled triangle, the square on the hypotenuse equals the sum of squares on the other two sides: a² + b² = c². It applies only to right-angled triangles. GCSE questions extend it to 3D problems, isosceles triangles split into two right-angled halves, and finding distances between coordinates.
- Quadratic formulaThe quadratic formula solves any quadratic equation of the form ax² + bx + c = 0: x = (−b ± √(b² − 4ac)) / 2a. Use it when factorising won't work cleanly. The discriminant b² − 4ac determines the number of real roots: positive → two distinct real roots; zero → one repeated root; negative → no real roots. GCSE Higher tier expects exact answers using surds.
- SOHCAHTOASOHCAHTOA is the mnemonic for the three right-angled trig ratios: Sine = Opposite / Hypotenuse, Cosine = Adjacent / Hypotenuse, Tangent = Opposite / Adjacent. Use it in any right-angled triangle to find an unknown side or angle when you know one side + one angle (other than the right angle). For non-right-angled triangles, use the sine rule or cosine rule instead.
- SurdsA surd is an irrational root that cannot be expressed exactly as a fraction — e.g. √2, √3, ∛5. GCSE Higher tier examines surd manipulation: simplifying (√50 = 5√2), rationalising the denominator (multiplying top and bottom by the conjugate), and exact-form answers in trigonometry, Pythagoras and quadratics. Decimal approximations lose marks where the question requires 'exact form'.
Maths — Both
- Cosine ruleThe cosine rule relates the three sides and one angle of any triangle: a² = b² + c² − 2bc · cos A. Use it when you know two sides and the included angle (SAS), or all three sides and want to find an angle (SSS). For right-angled triangles, Pythagoras' theorem (cos 90° = 0) is the natural special case.
- Sine ruleThe sine rule relates the sides and opposite angles of any triangle: a / sin A = b / sin B = c / sin C. Use it when you know two angles and a side (AAS or ASA) or two sides and a non-included angle (SSA, the ambiguous case). For right-angled triangles, SOHCAHTOA is simpler and preferred.
Physics — Both
- Conservation of momentumThe principle of conservation of momentum states that the total momentum of an isolated system remains constant, provided no external forces act. For two colliding objects: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. The principle applies to all collisions; kinetic energy is only conserved in elastic collisions. A-Level Physics also examines impulse: F·Δt = Δp (change in momentum), and 2D momentum problems where vectors must be resolved into components.
- Force diagramsA force diagram (or free-body diagram) shows all forces acting on a single object as arrows with direction and labelled magnitude. Used to apply Newton's laws to mechanics problems. For an object at rest, forces sum to zero (equilibrium). For an accelerating object, the resultant force ΣF = ma. GCSE Physics examines simple cases (object on a ramp, suspended object); A-Level extends to resolving forces into components on inclined planes and circular-motion problems.
- Hooke's LawHooke's Law states that the force required to extend (or compress) a spring is proportional to the extension, provided the spring is within its elastic limit: F = kx, where k is the spring constant (N/m) and x is the extension from natural length. Past the elastic limit the relationship becomes non-linear and the spring may deform permanently. A force-extension graph rewards correct gradient calculation (gradient = k).
- Ohm's LawOhm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided temperature is constant: V = IR. Where V is potential difference in volts, I is current in amps, and R is resistance in ohms. The law holds for ohmic conductors (most metallic conductors at constant temperature); non-ohmic components like filament bulbs or diodes do not obey it. A-Level Physics extends to internal resistance: V = ε − Ir.
- RefractionRefraction is the change in direction of a wave (typically light or sound) when it crosses a boundary between two media of different propagation speeds. Snell's Law quantifies it: n₁ sin θ₁ = n₂ sin θ₂, where n is the refractive index. Total internal reflection happens when light moves from a denser to less-dense medium and hits the boundary at or above the critical angle. A-Level Physics extends to fibre optics and thin-film interference.
- Specific heat capacitySpecific heat capacity (c) is the energy required to raise the temperature of 1 kg of a substance by 1 °C (or 1 K). The defining equation is ΔE = mcΔθ where m is mass in kg, c is in J/(kg·°C) and Δθ is temperature change. Water has c = 4181 J/(kg·°C); aluminium is around 900; lead is around 130. The substance with the higher c needs more energy for the same temperature rise.
- Work and energyWork is energy transferred when a force moves an object: W = F·s (when force and displacement are parallel) or W = F·s·cosθ. Measured in joules. The work-energy theorem states that the net work done on an object equals its change in kinetic energy: W = ΔKE = ½m(v² − u²). GCSE Physics introduces it via simple force × distance calculations; A-Level extends to integration when force varies with position (∫F·ds).
Physics — A-Level
- Electromagnetic inductionElectromagnetic induction is the generation of an electromotive force (EMF) in a conductor when the magnetic flux through it changes. Faraday's law: ε = −dΦ/dt, where Φ is magnetic flux linkage. Lenz's law (the minus sign) states that the induced EMF opposes the change causing it (conservation of energy). Applications: dynamos, transformers, induction cookers, eddy-current braking. AQA, Edexcel and OCR all examine flux-linkage graphs and the derivation of induced EMF.
- Internal resistanceInternal resistance (r) is the resistance inside a cell or battery that opposes current flow. The terminal potential difference is lower than the cell's electromotive force (EMF, ε) when current flows: V = ε − Ir. As current draw increases, terminal V drops. A-Level Physics examines this via plotting terminal V against I — the y-intercept is the EMF, the negative gradient is the internal resistance. The cell is also a closed loop in Kirchhoff analysis.
- Kirchhoff's lawsKirchhoff's laws govern electrical networks. The Current Law (KCL): the sum of currents entering a junction equals the sum of currents leaving it (conservation of charge). The Voltage Law (KVL): the sum of EMFs around a closed loop equals the sum of potential differences (conservation of energy). A-Level Physics applies them to circuits with multiple loops, branched paths and cells in parallel or series.
- Projectile motionProjectile motion describes the path of an object moving under gravity alone after being launched at an angle. Horizontal motion has constant velocity; vertical motion has constant acceleration g downwards. Resolve initial velocity into horizontal (v cosθ) and vertical (v sinθ) components, then apply suvat to each component independently. Range, maximum height and time of flight are the three exam-favourite calculations.
- Simple harmonic motionSimple harmonic motion describes oscillation where the restoring force is proportional to displacement and directed toward equilibrium: a = −ω²x. The system oscillates with constant period T = 2π/ω regardless of amplitude. SHM examples: mass on a spring (ω = √(k/m)), simple pendulum (ω = √(g/L) for small angles), molecular vibrations. A-Level Physics examines displacement, velocity and acceleration equations, energy in SHM and damping.
Revision technique — Both
Sciences — Both
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