1,800+ MCAT terms, defined.

Poverty is a lack of the resources needed for basic needs or ordinary social participation. It is defined in two ways: absolute (below a fixed standard) and relative (far less than others in the same society). Greater poverty usually goe...

Segregation is Mendel's first law: the two alleles of a gene separate during gamete formation, so each gamete receives one. Its physical basis is homologous chromosomes separating in meiosis I. Most testable: Aa × Aa gives 1:2:1 genotype...

Cross-sectional area is the area of the face exposed when a pipe, vessel, or wire is cut perpendicular to the flow or current. For a steady incompressible fluid, Av is constant, so a smaller area means faster flow. In a wire of fixed len...

A domain is a compact region of a single polypeptide that folds into a stable unit and often carries a specific function, such as binding a ligand, binding DNA, spanning a membrane or catalysis. Many domains can fold independently, so de...

Sigmund Freud founded psychoanalysis, the psychodynamic view that behavior is shaped by unconscious conflict among the id, ego, and superego. The most testable points are identifying his defense mechanisms in a vignette and knowing his p...

A carbonyl group is a carbon double-bonded to oxygen (C=O). Oxygen pulls electron density, so the carbon is partially positive and attacked by nucleophiles. This is the core reactivity of aldehydes, ketones, carboxylic acids and their de...

A net ionic equation shows only the species that actually change in a solution reaction, after removing spectator ions that appear unchanged on both sides. Aqueous strong electrolytes are written as ions; solids, gases, and weak electrol...

Formal operational is Piaget's last stage of cognitive development, starting around age 11 or 12, when a person can reason abstractly and hypothetically and test ideas systematically. Key contrast: concrete operational logic is tied to r...

Long-term memory is the relatively lasting store of information, with a very large capacity. It divides into explicit (episodic, semantic) and implicit (procedural, priming, conditioning) memory. Testable: elaborative rehearsal helps mov...

Meissner's corpuscle is an encapsulated mechanoreceptor in the upper dermis of hairless (glabrous) skin, such as fingertips and lips. It detects light touch and low-frequency vibration. It adapts rapidly, so it signals changes in touch,...

A monosaccharide is a single sugar unit, a polyhydroxy aldehyde (aldose) or ketone (ketose) with the formula (CH₂O)n, usually n = 3 to 7. It cannot be hydrolyzed into smaller sugars. Most testable: glucose and galactose are aldohexoses,...

Filtrate is the liquid that passes through the filter during filtration, carrying the solvent and dissolved solutes; the solid left on the filter is the residue. Whether the product is in the filtrate depends on the step, as in hot filtr...

Partial pressure is the pressure one gas in a mixture would exert if it alone filled the container at the same temperature. For ideal gases it equals the gas's mole fraction times the total pressure, and the partial pressures add up to t...

Laminar flow is smooth fluid motion in parallel layers (streamlines) that slide past one another without mixing. It is favored at low Reynolds number (roughly below 2000 in a pipe), and Poiseuille's law describes it. Higher Re leads to t...

Sensorimotor is Piaget's first stage of cognitive development, lasting from birth to about age 2, in which infants learn through their senses and motor actions. The most testable point is object permanence, the understanding that objects...

Socialization is the lifelong process by which people learn and internalize the norms, values, language, and roles of their society. The MCAT most often tests the agents of socialization (family, peers, school, media) and primary versus...

Magnification (m) is the ratio of image height to object height, m = hi/ho = −i/o. Its sign gives orientation (negative = inverted, positive = upright) and its magnitude gives size (|m| > 1 enlarged, |m| < 1 reduced).

The density of water is its mass per unit volume, about 1.00 g/mL or 1.00 × 10³ kg/m³ near room temperature. The most testable point is that ice is less dense than liquid water, because hydrogen bonds hold it in an open lattice, so ice f...

Configurational isomers are stereoisomers with the same connectivity but a different fixed 3D arrangement, so interconverting them requires breaking a bond. They include enantiomers and diastereomers (including cis/trans). Rotation about...

In biochemistry, aromatic usually describes amino acids whose side chains contain a planar ring of delocalized π electrons: phenylalanine, tyrosine, and tryptophan. They mostly sit in protein interiors, and Trp and Tyr absorb UV light ne...

Sublimation is a defense mechanism in Freud's psychodynamic theory in which an unacceptable impulse, such as aggression or sexual desire, is redirected into a socially acceptable activity. It is usually classed as a mature, adaptive defe...

Mendelian inheritance is the pattern of single-gene traits whose alleles segregate into separate gametes and, for unlinked genes, assort independently. Most testable: with complete dominance, Aa × Aa gives a 3:1 phenotype ratio and a 1:2...

The visual pathway carries signals from the retina via the optic nerve, optic chiasm, and thalamus (LGN) to the primary visual cortex in the occipital lobe. Testable point: nasal retinal fibers cross at the chiasm, so each visual field i...

A buffer solution contains a weak acid and its conjugate base (or a weak base and its conjugate acid) and resists large pH changes when small amounts of strong acid or base are added. Its pH depends on the pKa and the base-to-acid ratio,...

HPLC (high-performance liquid chromatography) separates a mixture by pumping a liquid mobile phase at high pressure through a column packed with fine stationary-phase particles. Compounds that stick more strongly to the stationary phase...

Feature detection is the idea that specific neurons in the visual cortex respond to particular stimulus features, such as edges, orientation, or motion, and later areas combine these into whole objects. It is a bottom-up process, and the...

Isocitrate is a six-carbon citric acid cycle intermediate formed from citrate by aconitase. Isocitrate dehydrogenase converts it to α-ketoglutarate, releasing CO₂ and making NADH; this is a key regulated, rate-limiting step of the cycle.

Adhesion is the attraction between molecules of different substances, such as water and glass. Cohesion is the attraction between like molecules. When adhesion exceeds cohesion, a liquid wets the wall, forms a concave meniscus and rises...

Atomic weight is the average mass of an element's atoms as found in nature, weighted by isotope abundance, in atomic mass units (u). It is the periodic table value and equals molar mass in g/mol. Testable point: it is a weighted average,...

The sensorimotor stage is the first stage in Piaget's theory of cognitive development, lasting from birth to about age 2. Infants learn by acting on the world with their senses and movements. Its key milestone is object permanence, knowi...

A reporter gene encodes an easily detected product, such as GFP, luciferase, or β-galactosidase (lacZ). It is linked to a promoter or to a gene of interest to show when and where that gene is expressed. The signal reflects the activity o...

A mesylate is the methanesulfonate group, CH₃SO₂O– (OMs), made from an alcohol and mesyl chloride. It turns a poor leaving group (OH) into an excellent one. Making it keeps the stereochemistry of the carbon, so a following SN2 gives net...

A fad is a short-lived behavior, style, or object that spreads quickly through a group by imitation and then fades. It is a form of collective behavior: spontaneous and unorganized, with no goal of changing society, which separates it fr...

Efficiency is useful output divided by total input (work or energy), e = Wout/Win, often given as a percentage. Real machines fall below 100% because some input is dissipated as heat; a heat engine's maximum is the Carnot value, 1 − TC/T...

Bond order is the number of electron pairs shared between two bonded atoms: 1 for single, 2 for double, 3 for triple bonds. Between the same two atoms, a higher bond order generally means a shorter, stronger bond. Resonance and MO theory...

A prosthetic group is a non-amino-acid cofactor, such as heme or FAD, that stays tightly bound to a protein, often covalently, and is needed for its function. Unlike a loosely bound coenzyme such as NAD⁺, it usually remains attached thro...

Incentive theory says behavior is motivated by external rewards or the avoidance of punishments that pull a person toward a goal, rather than by an internal need pushing. The testable contrast is with drive-reduction theory, though many...

Nicotine is a stimulant in tobacco that binds nicotinic acetylcholine receptors, raising dopamine in the brain's reward pathway. The MCAT uses it as a model of addiction: reinforcement, tolerance, and withdrawal keep use going.

The skeletal system is the bones, cartilage, and joining ligaments that support the body, protect organs, give muscles levers for movement, store calcium and phosphate, and house marrow. Testable point: bone is living, remodeled tissue t...

The Gabriel synthesis makes a primary amine from a primary alkyl halide: the phthalimide anion attacks by SN2, then hydrolysis or hydrazine releases R–NH₂. Its testable advantage is that it avoids the overalkylation seen when NH₃ is alky...

Threshold frequency (f₀) is the minimum frequency of light that can eject electrons from a particular metal in the photoelectric effect. Below f₀ no electrons are emitted, however intense the light. Above f₀, the extra photon energy beco...

An Arrhenius acid raises the H⁺ (H₃O⁺) concentration in water; an Arrhenius base raises the OH⁻ concentration in water. It is the narrowest acid-base definition, so NH₃ is a Brønsted-Lowry base but not an Arrhenius one. The name also tag...

Jean Piaget proposed a stage theory of cognitive development in which children build schemas through assimilation and accommodation. The stages occur in a fixed order: sensorimotor, preoperational, concrete operational, formal operationa...

An mRNA transcript is the RNA copy of a gene, made by RNA polymerase, that ribosomes read to build a polypeptide. It is complementary to the template strand and matches the coding strand with U in place of T. Eukaryotic transcripts are p...

Self-identities are the specific roles, traits, and group memberships (such as student, daughter, athlete, or a gender, racial, or religious group) that a person uses to define themselves. Each is one part of the self-concept, and the MC...

A lactone is a cyclic ester: the carbonyl carbon and the ester's single-bonded oxygen are both ring atoms (the C=O oxygen is outside the ring). It forms when a hydroxy acid esterifies intramolecularly, and five- and six-membered (γ and δ...

Circular motion is motion along a circular path. Even at constant speed the velocity direction keeps changing, so there is a centripetal acceleration a = v²/r toward the center. Most testable: centripetal force is not a new force. It is...

Reactants are the substances on the left of the arrow in a chemical equation. They are consumed as bonds break and atoms rearrange into products. Most testable: the limiting reactant, the one that runs out first, sets the maximum amount...

Wernicke's area is a region of the posterior temporal lobe, in the left hemisphere for most people, that supports language comprehension. Damage causes Wernicke's aphasia: fluent but meaningless speech with poor comprehension, unlike Bro...

The thyroid is a neck endocrine gland that secretes T3 and T4, which raise basal metabolic rate, and calcitonin, which lowers blood calcium. Most testable: the TRH → TSH → T3/T4 negative feedback loop, where low T4 with high TSH means a...