1,800+ MCAT terms, defined.

The normal line is an imaginary line perpendicular (90°) to a surface at the point where a light ray hits it. All angles of incidence, reflection and refraction are measured from it, not from the surface itself.

Benzoic acid, C₆H₅COOH, is a benzene ring bearing a carboxyl group; it is a weak acid (pKa about 4.2). Its conjugate base, benzoate, is resonance-stabilized and water soluble, so aqueous base pulls it out of an organic layer in an extrac...

The inner mitochondrial membrane is the folded inner membrane (cristae) that holds the electron transport chain and ATP synthase. It is largely impermeable to H⁺, so the chain's proton pumping builds a gradient across it. The testable po...

Prospective memory is remembering to carry out an intended action in the future, such as mailing a letter or taking a pill at a set time. It contrasts with retrospective memory, recall of past events or information. Testable point: it re...

Bacterial genetics covers how bacteria store, express, and exchange genes: usually one circular chromosome plus optional plasmids. Variation comes from mutation and horizontal transfer (transformation, transduction, conjugation), not mei...

Retinal disparity is the small difference between the images that fall on the two retinas because the eyes sit apart. The brain uses it as a binocular depth cue: the larger the disparity, the closer the object, and distant objects produc...

To neutralize is to react an acid with a base until the moles of H⁺ supplied equal the moles of OH⁻ (or other base) supplied. The result is pH 7 at 25 °C only for a strong acid with a strong base. With a weak acid or weak base, the equiv...

Visible light is the band of electromagnetic waves the human eye detects, roughly 400 nm (violet) to 700 nm (red). Violet has the shortest wavelength and highest frequency and energy; red has the longest wavelength and lowest. All colors...

Electron density is how much electron probability sits in a region of a molecule. It is greater near electronegative atoms, lone pairs, and π systems, and lower near electron-withdrawing groups. Electron-rich sites tend to act as nucleop...

In labeling theory, secondary deviance is continued rule-breaking after a person is labeled deviant and adopts that label as part of their identity. Primary deviance is the initial act. The testable point: in this account, society's reac...

Pancreatic means "of the pancreas." It is a modifier, not a single concept. In biochemistry it usually points to pancreatic digestive enzymes (amylase, lipase, trypsin and other proteases), secreted mostly as inactive zymogens into the s...

Charles' law says that for a fixed amount of an ideal gas at constant pressure, volume is directly proportional to absolute temperature: V₁/T₁ = V₂/T₂. Heating makes the gas expand. The testable trap is that T must be in kelvins, not °C.

A diopter (D) is the unit of optical power of a lens or curved mirror, equal to 1/f with f in meters (1 D = 1 m⁻¹). Shorter focal length means greater power; converging lenses are positive, diverging negative. Powers of thin lenses in co...

Agents of socialization are the groups and institutions through which people learn the norms, values, language, and roles of their society. The main ones are family, peers, school, mass media, and religion; family is usually the primary...

A bacterial growth curve plots the log of viable cell number against time in a closed batch culture. It has four phases: lag, exponential (log), stationary, and death. Most testable: in exponential phase, binary fission gives a constant...

Hydrophilic ("water-loving") groups are polar or charged functional groups, such as −OH, −NH₂, −COOH and −COO⁻, that interact favorably with water through hydrogen bonds or ion–dipole forces. Adding them generally raises a molecule's sol...

Behavioral genetics studies how genetic and environmental differences among people contribute to differences in behavior and traits, using twin, adoption and family studies. Most testable: heritability describes variation within a popula...

A photon's energy is the energy carried by one quantum of light, E = hf = hc/λ. It rises with frequency and falls with wavelength, so a UV photon carries more energy than a red or infrared one. Brightness changes the number of photons, n...

Maximum kinetic energy is the largest kinetic energy of electrons ejected from a metal in the photoelectric effect. It equals the photon energy minus the work function, Kmax = hf − φ. It rises with light frequency, not intensity.

Sanger sequencing reads a DNA sequence by copying a template with polymerase in the presence of dideoxynucleotides (ddNTPs), which lack a 3′-OH and stop chain growth. The fragments are separated by size, and the gel is read from smallest...

Collective behaviour is relatively spontaneous, unstructured action by a large group, such as a crowd, panic, riot, or fad, not guided by established institutional norms. The MCAT tests three explanations: contagion, convergence, and eme...

Bone remodelling is the continual turnover of bone: osteoclasts resorb old matrix and osteoblasts deposit new matrix. It repairs bone, adapts it to load and exchanges calcium with blood. PTH raises blood Ca²⁺ by promoting resorption; cal...

Consciousness-altering (psychoactive) drugs change awareness, mood, or perception by altering neurotransmitter activity at synapses. The MCAT sorts them into depressants, stimulants, opiates, and hallucinogens, and tests which neurotrans...

A molecular vibration is the repeating stretching or bending of bonds about their equilibrium positions, with quantized energies. It is what IR spectroscopy detects: a vibration absorbs IR when it changes the dipole moment, and stronger...

Standard free energy (ΔG°) is the Gibbs free energy change for a reaction when all species are in their standard states (1 M solutes, 1 atm gases, usually 298 K). It fixes Keq: ΔG° < 0 means Keq > 1. Direction in real conditions depends...

Critical speed is the fluid speed above which smooth laminar flow in a tube becomes turbulent. It rises with viscosity and falls with fluid density and tube diameter. Turbulence is likely when the actual speed exceeds it.

The dependency ratio compares people usually presumed too young or too old to work (under 15 and 65 and over) with working-age people (about 15–64). It is based on age alone, and a high value can come from many children or many older adu...

Taq polymerase is a heat-stable DNA polymerase from the hot-spring bacterium Thermus aquaticus. It survives the roughly 95 °C denaturation step of PCR, so it does not need to be re-added each cycle. The testable point is that PCR uses it...

Gender as a social construct means the roles, behaviors, and meanings a society attaches to being male, female, or another gender are created and taught by that society, not set by biology alone. Sex is biological; gender norms vary acro...

Acrylic acid is propenoic acid, CH₂=CH–COOH, the simplest carboxylic acid with a C=C conjugated to the C=O (an α,β-unsaturated acid). It is slightly more acidic than propanoic acid, and its alkene can undergo addition or polymerization.

The principal quantum number, n, is a positive integer (1, 2, 3, …) that labels an electron's shell and sets the orbital's size and main energy level. Higher n means higher energy and a larger average distance from the nucleus. (Standard...

Sound intensity is the power a sound wave delivers per unit area perpendicular to its travel, in W/m². The most testable point: for a point source radiating equally in all directions it falls as 1/r², and it is proportional to amplitude...

Capillary exchange is the movement of gases, nutrients, wastes and fluid between capillary blood and interstitial fluid. Solutes diffuse down gradients, while fluid shifts with the balance of hydrostatic pressure (out) and oncotic pressu...

In Erikson's psychosocial theory, generativity vs. stagnation is the crisis of middle adulthood: contributing to and guiding the next generation (generativity) versus self-absorption and feeling unproductive (stagnation). Testable point:...

A glycosidic bond joins a sugar's anomeric carbon to an oxygen (or, in nucleosides, a nitrogen) of another molecule, formed by condensation. Most testable: α versus β configuration decides enzyme specificity, so humans digest α-1,4 starc...

Deposition is the phase change in which a gas becomes a solid directly, skipping the liquid phase. It is the reverse of sublimation, so it is exothermic and lowers entropy. On a phase diagram it is the gas-to-solid crossing of the solid–...

Archimedes' principle says the upward buoyant force on an object in a fluid equals the weight of the fluid it displaces. Most testable point: the force depends on the fluid's density and the volume displaced, not on the object's own mass.

Group polarization is the tendency of group discussion to push a shared initial leaning to a more extreme position than the members' average view before talking. Two accounts explain it: social comparison (normative) and persuasive argum...

An alkyl chain is a saturated, nonpolar hydrocarbon group of single-bonded carbons and hydrogens (methyl, ethyl, propyl) attached to the rest of a molecule. Longer chains generally increase hydrophobicity and London dispersion forces, so...

A half reaction is the oxidation or the reduction step of a redox reaction, written alone with the electrons shown. Oxidation occurs at the anode and reduction at the cathode, and the two add, with electrons cancelled, to give the overal...

Coulomb's law gives the electrostatic force between two point charges. It is proportional to the product of the charges and inversely proportional to the square of their separation, so doubling the distance cuts the force to one quarter....

Cell signalling is how a cell detects an extracellular signal (a ligand) and turns it into a response: reception, transduction, response. Most testable: water-soluble ligands typically bind surface receptors and use second messengers lik...

Identity formation is the process of building a stable sense of who you are, shaped by personal exploration and social influences. The MCAT ties it to Erikson's identity vs. role confusion in adolescence and to Marcia's four identity sta...

A deoxy sugar is a carbohydrate in which a hydroxyl group has been replaced by a hydrogen. The standard example is 2-deoxyribose in DNA, which lacks the OH on C2. The most testable point: the missing 2′-OH makes DNA's backbone more resis...

An amine is a nitrogen with a lone pair bonded to hydrogen and/or alkyl or aryl groups (RNH₂, R₂NH, R₃N). The lone pair makes it a base and a nucleophile, so below the pKa of its conjugate acid (about 10–11 for simple alkylamines) it exi...

In Erikson's psychosocial theory, identity vs. role confusion is the adolescent crisis (roughly ages 12 to 18) of forming a coherent sense of self. Success gives a stable identity (virtue: fidelity); failure leaves uncertainty about who...

Electric current is the rate at which charge flows past a point, measured in amperes (coulombs per second). By convention it points the way positive charge would move, from high to low potential, which is opposite to electron flow in met...

Impression formation is the process of combining cues about a person, such as appearance, behavior and traits, into one overall judgment. The most testable point is that information is not weighed equally: early information often counts...

Ammonia (NH₃) has a nitrogen lone pair; an amine replaces one or more H with alkyl or aryl groups. Both are weak bases and nucleophiles that form ammonium ions with acid, and delocalizing the lone pair (aniline, amides) lowers basicity.

A disaccharide is two monosaccharides joined by a glycosidic bond, formed by dehydration (loss of H₂O). Maltose, lactose and sucrose are the standard examples. Most testable: a disaccharide is reducing when an anomeric carbon is still fr...