1,800+ MCAT terms, defined.

This is the bulk modulus (B), the pressure change needed per fractional change in volume. A large B means the medium is hard to compress. The testable point is that sound speed is v = √(B/ρ), so stiffer media such as solids and liquids u...

The primary auditory cortex is the region of the temporal lobe (Heschl's gyrus) that first receives sound information from the thalamus, specifically the medial geniculate nucleus. It is organized tonotopically, so different pitches map...

Slow-wave sleep is the deepest stage of non-REM sleep (stage N3, formerly stages 3 and 4), marked by high-amplitude, low-frequency delta waves. It is the stage from which a sleeper is hardest to wake, and it is most abundant early in the...

An s orbital is a spherically symmetric electron probability distribution around the nucleus, with angular momentum quantum number ℓ = 0. Each s subshell has one orbital holding at most 2 electrons, and it grows larger and higher in ener...

When one object exerts a force on a second, the second exerts a force of equal magnitude and opposite direction on the first. The two forces act on different objects, so they do not cancel when you find the net force on either one.

Conjugated π bonds are double or triple bonds separated by one single bond, so adjacent p orbitals overlap and the π electrons are delocalized over the whole chain. This lowers energy (extra stability) and shifts UV-Vis absorption to lon...

Okazaki fragments are short pieces of new DNA, each started by an RNA primer, that are made on the lagging strand during replication. DNA polymerase only extends a 3' end, so the lagging strand is built discontinuously and the pieces are...

A negative control is a group or condition that does not get the active manipulation, such as a placebo or no-treatment group, and is expected to show no effect. If it changes anyway, something other than the independent variable is infl...

Chromosomal abnormalities are changes in chromosome number (aneuploidy such as trisomy or monosomy, usually from nondisjunction) or in structure (deletion, duplication, inversion, translocation). Most testable: nondisjunction in meiosis...

Post-conventional is the third and highest level of Kohlberg's moral development. Right and wrong are judged by abstract, self-chosen principles such as justice and human rights, which can override laws and social norms. The exam scores...

The equilibrium point is the state of a reversible reaction in which forward and reverse rates are equal, so concentrations stay constant at a given temperature while both reactions continue. The most testable point: at equilibrium Q = K...

Perceived loudness is how strong a sound seems to a listener. It increases with sound intensity (which scales with amplitude squared), but on a logarithmic scale, so it is tracked by the decibel level β = 10 log(I/I₀). Every 10 dB is a t...

To tautomerize is to convert between tautomers, constitutional isomers that interconvert by moving a hydrogen and shifting a double bond, as in keto-enol. For simple aldehydes and ketones the keto form usually dominates, and the planar e...

The sympathetic nervous system is the division of the autonomic nervous system that mobilizes the body for threat or exertion ("fight or flight"). Heart rate, breathing, and alertness rise while digestion slows. It generally opposes the...

A knockout is an organism or cell line in which a chosen gene has been deliberately disabled or deleted, so it makes no functional product. Comparing it with the wild type shows what the gene normally does. The key inference is loss of f...

STP means standard temperature and pressure: 0 °C (273 K) and 1 atm. At STP, one mole of any ideal gas occupies 22.4 L, so you can convert directly between gas volume and moles without using the full ideal gas law.

Slit width (a) is the size of the opening a wave passes through in single-slit diffraction. The narrower the slit, the more the wave spreads, so the central bright band gets wider. Dark fringes obey a sin θ = mλ.

Depth cues are visual signals the brain uses to judge distance and three-dimensional layout from the retinal image. They are binocular (need both eyes, e.g., retinal disparity, convergence) or monocular (work with one eye, e.g., linear p...

An endocrine gland is a ductless gland that secretes hormones into the surrounding fluid and blood, which carry them to distant target cells. Only cells with the matching receptor respond, so target specificity depends on receptors, not...

A competitive inhibitor resembles the substrate and usually binds the enzyme's active site reversibly, so it competes with substrate. It raises the apparent Km but leaves Vmax unchanged, because enough substrate can outcompete the inhibi...

Erikson's theory of psychosocial development describes eight lifespan stages, each built around a social crisis between a positive and a negative outcome, such as trust vs mistrust. The most testable point is matching a described behavio...

The ideal gas equation, PV = nRT, links the pressure, volume, moles and absolute temperature of a gas modeled as having negligible particle volume and no intermolecular forces. The most testable point is that T must be in kelvins. Real g...

A conservative force does work that depends on the start and end points, not the path, so the net work over any closed loop is zero. Gravity, spring forces and electrostatic forces are examples. Friction and air resistance are not, so th...

A soap is the sodium or potassium salt of a long-chain fatty acid, made by base hydrolysis (saponification) of triglycerides. Its charged head and nonpolar tail make it amphipathic, so it forms micelles that trap grease in a nonpolar cor...

Memory errors are lapses of forgetting (failed encoding, decay, interference, retrieval failure) and distortions from reconstructive recall, such as the misinformation effect. Most testable: proactive interference is old material impairi...

The parasympathetic nervous system is the branch of the autonomic nervous system that dominates in calm conditions ("rest and digest"). It slows heart rate, constricts the pupils and promotes digestion, generally opposing the sympathetic...

Gram staining is a differential stain that sorts bacteria by cell wall structure. Gram-positive cells have a thick peptidoglycan wall and stay purple; gram-negative cells have a thin wall plus an outer membrane and appear pink-red after...

A dehydration reaction loses water: either an alcohol eliminates H₂O to form an alkene (acid, heat), or two molecules join with H₂O released (condensation). Most testable: the more substituted alkene (Zaitsev product) usually dominates,...

An exponent tells how many times a base is multiplied by itself. In general chemistry it appears in powers of ten, equilibrium expressions (coefficients become exponents) and rate laws (reaction orders). Most testable: exponents add when...

A binocular cue is a depth cue that needs both eyes. The two main ones are retinal (binocular) disparity, the slight difference between the two retinal images, and convergence, the inward turning of the eyes for near objects. Monocular c...

Sphingosine is an 18-carbon amino alcohol with a long hydrocarbon chain. It is the backbone of sphingolipids, used in place of glycerol. Adding a fatty acid to its amino group by an amide bond gives ceramide, the precursor of sphingomyel...

In Marx's conflict theory, class consciousness is a group's awareness of its shared economic position and of interests that oppose another class, such as workers recognizing exploitation by owners. Its opposite is false consciousness, wh...

A dioic acid is a dicarboxylic acid, named by adding "-dioic acid" to the alkane name, as in butanedioic acid, HOOC–CH₂–CH₂–COOH. Both carboxyl carbons are part of the chain count. Testable point: pKa1 is lower than pKa2, and a nearby se...

SO₄²⁻ is the sulfate ion, a tetrahedral polyatomic anion with sulfur in the +6 oxidation state. It is the conjugate base of HSO₄⁻ and a very weak base. Most sulfate salts dissolve, but BaSO₄ and PbSO₄ are the classic insoluble exceptions.

Damping is the loss of mechanical energy from an oscillating system, usually as thermal energy through friction or drag, so the amplitude decays over time. Most testable: with light damping, amplitude and energy fall but the frequency ch...

Cultural assimilation is the process by which a person or group adopts the language, values, and norms of a dominant culture, so original cultural traits fade. Most testable point: it differs from multiculturalism (pluralism), where cult...

A multipotent cell is a stem cell that can differentiate into several cell types, but generally only those of one tissue or lineage. The classic example is the hematopoietic stem cell, which gives rise to the blood cell types. Its potenc...

A conjugated protein is a protein that contains a non-amino-acid component, called a prosthetic group, bound to its polypeptide chain. The most testable point is that the prosthetic group is often required for function, as with heme in h...

A reaction is zero order in a reactant when its rate does not depend on that reactant's concentration, so rate = k. The most testable points: [A] versus time is a straight line with slope −k, and k has units of M/s.

Diamagnetic materials have no permanent atomic magnetic moments, usually because their electrons are all paired, and they are weakly repelled by an external magnetic field. The field induces a small opposing moment. Unpaired electrons po...

Depth perception is the ability to see the world in three dimensions and judge distance, even though each retina receives a flat image. The brain uses binocular cues (retinal disparity, convergence) and monocular cues (such as linear per...

E2 is bimolecular elimination: in one concerted step, a strong base removes a β-hydrogen as the leaving group departs, forming an alkene. The rate depends on both substrate and base, and the H and leaving group must be anti-periplanar.

A gender script is a culturally learned guide to how people of a given gender are expected to act, speak and respond in a particular situation, such as a date. Scripts are acquired through socialization, so the MCAT treats them as social...

The standard state is the agreed reference condition for thermodynamic values: pure solids and liquids, gases at 1 bar (older texts use 1 atm), solutes at 1 M, usually tabulated at 298 K. Values there carry a ° sign (ΔG°, E°); ΔG° relate...

Group polarization is the tendency for discussion to make a group's average attitude more extreme in the direction it already leaned. Two accounts explain it: persuasive arguments (informational influence) and social comparison (normativ...

Vesicles are small membrane-bound sacs that carry cargo within a cell, into it or out of it. They bud from one membrane and fuse with another, so the bilayer is conserved and the vesicle interior stays equivalent to the extracellular spa...

An enzyme-linked receptor is a membrane receptor whose intracellular domain is an enzyme or is bound to one. The classic example is the receptor tyrosine kinase, which autophosphorylates tyrosines after ligand binding and starts cascades...

An exocrine gland releases its secretion through a duct onto an epithelial surface or into a lumen, such as the skin or the gut, rather than into the blood. Most testable point: the pancreas is both. Acinar cells are exocrine (digestive...

Informational social influence is conforming because you treat other people's behavior as evidence of what is correct. It is strongest when a situation is ambiguous or you feel unsure, and it often leads to private acceptance. Normative...

A d orbital belongs to a subshell with angular momentum quantum number l = 2. Each d subshell has five orbitals holding up to 10 electrons and first appears at n = 3. Most testable: transition metals fill (n−1)d, and as cations they lose...