1,800+ MCAT terms, defined.

Chunking is an encoding strategy that groups separate items into larger meaningful units so more information fits in working memory. The limit applies to the number of chunks (classically about 7 ± 2), so grouping raises how much is held...

Calcium homeostasis is the negative-feedback control of blood Ca²⁺ within a narrow range. Low Ca²⁺ triggers parathyroid hormone (PTH), which raises it through bone, kidney and (via vitamin D) gut; high Ca²⁺ triggers calcitonin from the t...

A nitrile is an organic compound with a carbon triple-bonded to nitrogen, R–C≡N (the cyano group). The most testable point is that SN2 attack by cyanide on an alkyl halide adds one carbon, and the nitrile can then be hydrolyzed to a carb...

Methyl orange is a weak-acid pH indicator that is red below about pH 3.1 and yellow above about 4.4. Because its range is acidic, it suits titrations whose equivalence point is below 7, such as a strong acid titrated with a weak base.

Thermal energy is the energy of random particle motion in a substance, set by its temperature and by how much of it there is. Heat is thermal energy transferred because of a temperature difference. Testable point: temperature tracks aver...

Cognitive development is the change in thinking, language, and problem solving from infancy onward. The MCAT centers on Piaget's four stages (sensorimotor, preoperational, concrete operational, formal operational), with Vygotsky's social...

Counterconditioning is a classical-conditioning method that replaces an unwanted conditioned response with a new, incompatible one by pairing the stimulus with something that triggers the new response. It underlies systematic desensitiza...

A monatomic ion is a charged particle made from a single atom that has gained or lost electrons, such as Na⁺, Ca²⁺, Cl⁻ or O²⁻. Contrast it with a polyatomic ion like SO₄²⁻. Most testable: main-group charge follows from group position, a...

A primary alcohol has its –OH on a carbon bonded to one other carbon (R–CH₂–OH), as in ethanol; methanol is usually grouped with them. The most testable point: oxidation gives an aldehyde with PCC but a carboxylic acid with strong oxidan...

In Freud's later dual-instinct theory, Eros is the life instinct: the drive toward survival, pleasure, sexuality, and creation, powered by libido. It is paired with Thanatos, the death instinct, and the exam tests telling the two apart.

Chemical digestion is the enzyme-catalyzed hydrolysis of food macromolecules into small, absorbable units. Most of it occurs in the small intestine. The testable point is matching each enzyme to its site and pH, such as pepsin in the aci...

Ethnography is a qualitative research method in which a researcher immerses in a group or community, usually through participant observation over an extended time, to describe its culture from the members' view. It gives rich, in-depth d...

A polyatomic ion is a group of two or more covalently bonded atoms that carries a net charge, such as NO₃⁻, SO₄²⁻ or NH₄⁺. It behaves as one unit and generally stays intact when a salt dissolves, so fully dissociated Na₂SO₄ gives 2 Na⁺ a...

Beta-negative (β⁻) decay is a radioactive process in which a neutron in the nucleus turns into a proton and emits an electron (the beta particle) and an antineutrino. The mass number A stays the same and the atomic number Z rises by 1, s...

A bond angle is the angle between two bonds that share a central atom. VSEPR and hybridization set it: sp³ about 109.5°, sp² about 120°, sp 180°. Lone pairs push bonding pairs closer, so angles in NH₃ and H₂O are somewhat smaller than 10...

Medicalization is the process by which a normal life event or a behavior once seen as immoral or criminal comes to be defined and treated as a medical problem. The testable point is that it reflects changing social definitions, an exampl...

The matrix is the innermost compartment of a mitochondrion, enclosed by the inner membrane. It holds the pyruvate dehydrogenase complex and most citric acid cycle enzymes. Protons are pumped out of it during the electron transport chain,...

A polyvalent (polyprotic) acid can donate more than one proton per molecule, and a polyvalent base can accept more than one. Protons transfer stepwise, and each later step generally has a smaller Ka (Ka1 > Ka2 > Ka3), giving several buff...

Beta-positive (β+) decay is a nuclear decay in which a proton becomes a neutron, and the nucleus emits a positron and a neutrino. The atomic number Z falls by 1 and the mass number A stays the same. It typically occurs in proton-rich nuc...

Morphology is the study of how words are built from morphemes, the smallest units of language that carry meaning (such as un-, happy, -ness). The most testable point: a morpheme is not a phoneme, because phonemes are sound units that car...

Passive diffusion is the net movement of a solute directly across the lipid bilayer, down its concentration gradient, with no transport protein and no ATP. Its rate rises linearly with the gradient and does not saturate, unlike facilitat...

Fractional distillation separates miscible liquids with close boiling points using a fractionating column, where repeated condensation and revaporization enrich the vapor in the more volatile component. The lower-boiling liquid distills...

Pragmatics is the component of language concerned with how context, social norms, and nonverbal cues shape meaning beyond the literal words, such as sarcasm, indirect requests, and turn-taking. Testable point: it differs from semantics (...

Post-transition metals (poor metals) are the p-block metals right of the transition metals, such as Al, Ga, In, Sn, Tl, Pb and Bi. They are generally softer and lower-melting than transition metals, and the heavier ones often form ions w...

The dielectric constant κ is a dimensionless number for an insulator that tells how much it multiplies a capacitor's capacitance (C = κC₀) by weakening the field inside. Testable point: with the battery connected, V stays fixed and Q ris...

Phosphoglycerate mutase is the glycolytic enzyme that shifts the phosphate on glycerate from carbon 3 to carbon 2, converting 3-phosphoglycerate to 2-phosphoglycerate. It is an isomerase, so no ATP or NADH is made or used in this step, a...

A prodrome is the early period of subtle, milder changes that comes before the full symptoms of a disorder appear. On the MCAT it is most often tied to schizophrenia, where social withdrawal, odd beliefs and declining functioning can pre...

Rhodopsin is the light-sensitive pigment in the rods of the retina, made of the protein opsin bound to retinal (from vitamin A). It supports vision in dim light. Light hyperpolarizes the rod, so it releases less glutamate, and rhodopsin...

Smooth muscle contraction is the slow, usually involuntary shortening of nonstriated muscle. Ca²⁺ binds calmodulin, which activates myosin light-chain kinase to phosphorylate myosin, so it lacks the troponin step used in skeletal muscle.

Strain is the extra energy a molecule has because its geometry departs from the ideal, which makes it less stable. It comes from angle strain, torsional strain (eclipsing) and steric strain (crowding). Chair cyclohexane is nearly strain-...

The Rutherford model pictures the atom as a tiny, dense, positively charged nucleus holding nearly all the mass, with electrons in the mostly empty space around it. It came from the gold foil experiment, where most alpha particles passed...

The ampere (A) is the SI unit of electric current, the rate of charge flow: 1 A = 1 coulomb per second. The most testable point is that current in amperes follows I = V/R, so it depends on the voltage and the resistance in the circuit.

Thanatos is the death instinct in Freud's psychoanalytic theory, an unconscious drive toward aggression, destruction, and, when turned inward, self-destructive behavior. It is paired with Eros, the life instinct. Testable point: both ari...

Electron carriers are small molecules (NAD⁺/NADH, FAD/FADH₂, NADP⁺/NADPH) that accept electrons when a substrate is oxidized and donate them elsewhere. In catabolism NADH and FADH₂ deliver electrons to the electron transport chain, and N...

Xenocentrism is the preference for the products, styles, or ideas of a foreign culture over those of one's own. It is the opposite of ethnocentrism, which judges other cultures by one's own standards and favors one's own.

A primary (1°) alcohol has its OH on a carbon bonded to just one other carbon, written R–CH₂OH. The most testable point is oxidation: PCC stops at the aldehyde, while strong oxidants such as Jones reagent or KMnO₄ carry it on to the carb...

A solvation layer is the shell of solvent molecules oriented around a dissolved ion or polar solute by intermolecular attraction (a hydration shell in water). Key point: favorable solute–solvent attractions such as ion–dipole forces driv...

Electrostatic describes electric charges at rest and the forces, fields and potentials they produce. The most testable point is Coulomb's law: the force between two point charges falls off as 1/r², repelling for like charges and attracti...

Alertness is the waking state of consciousness in which a person is responsive to the environment and able to focus attention. On an EEG it appears mainly as beta waves (high frequency, low amplitude), and the reticular formation helps m...

Blood glucose control is the hormonal regulation of blood glucose within a narrow range by negative feedback. Insulin from pancreatic β cells lowers glucose after a meal, and glucagon from α cells raises it during fasting. Know the direc...

A Fischer projection is a flat drawing of a chiral molecule, such as a sugar or amino acid, with a vertical carbon chain, horizontal bonds toward the viewer and vertical bonds away. Most testable: D or L is read from the side of the OH (...

A titration indicator is a weak acid or base whose acid and conjugate-base forms differ in color, so its color change signals the endpoint of a titration. It works best when its pKa is near the pH of the equivalence point, which sits on...

A frequency shift is the change between the frequency a source emits and the frequency a detector measures, caused by their relative motion (the Doppler effect). Approach raises the observed frequency and separation lowers it, while the...

The blind spot is the optic disc, where the optic nerve leaves the retina. It has no rods or cones, so light falling on it is not detected. We usually do not notice it because the other eye covers the gap and the brain fills in the missi...

The chair conformation is the puckered, lowest-energy shape of cyclohexane, with bond angles near 109.5° and all C–C bonds staggered. Ring flip swaps axial and equatorial positions, and bulky substituents are generally more stable equato...

Incidence is the number of new cases of a condition that arise in a defined population during a specified time period. Prevalence counts all existing cases at a point in time, so incidence measures how fast new cases appear, while preval...

A double-displacement (metathesis) reaction swaps the partner ions of two compounds: AB + CD → AD + CB. It typically proceeds when a product leaves solution as a precipitate, a gas, or a weak electrolyte such as water. No oxidation numbe...

Electric charge is the property of matter that produces electric forces. It is positive or negative, measured in coulombs, quantized in units of e = 1.60 × 10⁻¹⁹ C, and conserved. Most testable: like charges repel, opposite charges attra...

Psychoanalytic theory, developed by Freud, holds that behavior and personality are shaped largely by unconscious drives and by conflicts, especially from early childhood. The id, ego, and superego conflict, and the ego uses defense mecha...

Cell junctions are specialized structures that connect neighboring animal cells or link a cell to the extracellular matrix. The three main types are tight junctions (seal and block paracellular leakage), desmosomes (anchor cells mechanic...