1,800+ MCAT terms, defined.

Memory is the encoding, storage, and retrieval of information. In the information-processing model, input passes from sensory to working to long-term memory, and long-term memory is explicit (episodic, semantic) or implicit (procedural,...

Temperature measures the average kinetic energy of the particles in a sample, and it sets the direction of heat flow (hot to cold). For an ideal gas, average translational kinetic energy is proportional to absolute temperature, so gas pr...

Resistance (R) is a measure of how strongly a conductor or circuit element opposes the flow of charge, in ohms (Ω). For an ohmic element V = IR, and R = ρL/A, so resistance rises with length and falls with cross-sectional area. Parallel...

An alcohol is an organic compound with a hydroxyl group (–OH) on a saturated sp³ carbon. Hydrogen bonding raises its boiling point. The most testable point is oxidation: primary gives an aldehyde or carboxylic acid, secondary gives a ket...

An operon is a cluster of genes, typical of prokaryotes, that share one promoter and operator and are transcribed together as one mRNA. The most testable point: the lac operon is inducible (allolactose releases the repressor), while the...

Stress is the physiological and psychological response to a demand or threat (a stressor) that a person appraises as taxing or exceeding their ability to cope. Most testable: appraisal matters, so the same event can be stressful for one...

Microtubules are hollow, polar cytoskeletal tubes made of α- and β-tubulin dimers. They form the mitotic spindle, the core of cilia and flagella, and tracks for motor proteins. Most testable: kinesin usually walks toward the plus end, dy...

Personality is the relatively stable pattern of thoughts, feelings, and behaviors that characterizes a person across time and situations. The MCAT tests it through competing perspectives (psychoanalytic, humanistic, trait, social cogniti...

Frequency is the number of complete cycles of a wave or oscillation per second, measured in hertz, and it is the inverse of the period. The most testable point: the source sets it, so frequency stays the same when a wave enters a new med...

A carboxylic acid is an organic compound with the carboxyl group, –COOH. It is a weak acid, yet far more acidic than an alcohol because its carboxylate conjugate base is resonance-stabilized. Nearby electron-withdrawing groups increase i...

An attitude is a learned, relatively stable evaluation of a person, object, event, or idea, with affective, behavioral, and cognitive (ABC) components. Attitudes can influence behavior, but behavior can also change attitudes, as cognitiv...

H₂O is water, a bent polar molecule that hydrogen bonds, dissolves ions and polar molecules, and acts as a reactant in hydrolysis or a product in condensation. Most testable: nonpolar groups cluster away from water (hydrophobic effect),...

Electrons are negatively charged subatomic particles that occupy quantized orbitals around the nucleus. Their arrangement, especially the valence electrons, sets bonding, periodic trends and redox behavior. Transition metal cations lose...

A magnetic field B is the vector field produced by moving charges (currents) and by magnets, measured in tesla. It exerts a force on a moving charge perpendicular to both v and B, so it changes the particle's direction but does no work a...

Functionalism is a macro-level sociological theory that sees society as a system of interdependent institutions, each serving functions that keep society stable. Its testable contrast is with conflict theory, which stresses power and ine...

Absorption is the movement of digested nutrients, water, and ions from the gut lumen across the intestinal epithelium into blood or lymph, mostly in the small intestine. Testable point: sugars and amino acids go to capillaries, but most...

An aldehyde is an organic compound whose carbonyl group (C=O) carbon is bonded to one hydrogen and one R group (RCHO), at the end of a chain. Its electrophilic carbonyl carbon is usually more reactive toward nucleophiles than a ketone's,...

Attention is the limited-capacity process of focusing awareness on some stimuli while giving others less processing. Most testable: models differ on where filtering happens (Broadbent early filter, Treisman attenuation, late selection),...

The equivalence point of a titration is where the moles of added titrant exactly equal the moles of analyte, as the reaction stoichiometry requires. Its pH is 7 for a strong acid with a strong base, above 7 for a weak acid with a strong...

Work is energy transferred to or from an object by a force acting through a displacement. Only the force component along the displacement counts, so W = Fd cosθ. Work is positive when the force has a component along the motion and negati...

Hydrolysis is the cleavage of a bond by water, with H added to one fragment and OH to the other. It is the reverse of condensation (dehydration synthesis). Testable point: it splits peptide, glycosidic, ester and ATP phosphoanhydride bon...

Norms are shared, often unwritten rules about how people are expected to behave in a society or group. The MCAT ranks them by seriousness: folkways (mild conventions), mores (morally significant), and laws (formally enforced), upheld by...

A ketone is an organic compound with a carbonyl group (C=O) whose carbon is bonded to two other carbon groups, R–CO–R′. The carbonyl carbon is electrophilic, so ketones undergo nucleophilic addition, and unlike aldehydes they resist mild...

Discrimination is the behavior of treating people differently, usually unfairly, because of their group membership. It is the action component of bias, separate from stereotypes (beliefs) and prejudice (attitudes), and it can be individu...

The CNS (central nervous system) is the brain and spinal cord, which integrate sensory input and generate motor output. Everything outside them is the peripheral nervous system (PNS). The autonomic and somatic systems are PNS divisions,...

Electric current is the rate at which charge flows past a point in a circuit, measured in amperes (1 A = 1 C/s). By convention it points the way positive charge would move, from high to low potential, even though electrons usually carry...

A base is a species that accepts a proton (Brønsted-Lowry), raises [OH⁻] in water (Arrhenius), or donates an electron pair (Lewis). Most questions use the Brønsted-Lowry view: a stronger base has a larger Kb and a weaker conjugate acid.

Conflict theory is a macro-level sociological view that society is shaped by competition among groups for scarce resources and power, so social order reflects the dominance of powerful groups. The testable contrast is with functionalism,...

The isoelectric point (pI) is the pH at which a molecule, such as an amino acid or protein, has no net charge. Below the pI it carries a net positive charge, and above it a net negative charge. For an amino acid with a nonionizable side...

Anxiety is a state of apprehension, tension and worry about a possible future threat, usually with physiological arousal. It is a disorder when excessive, persistent and impairing. The MCAT often asks you to tell generalized anxiety, pan...

An ester (RCOOR′) is a carboxylic acid derivative in which the acid's –OH is replaced by an alkoxy group (–OR′). Most testable: it forms from a carboxylic acid and an alcohol (Fischer esterification) and reverts by hydrolysis, and base h...

An electric field is the force per unit positive test charge that a source charge or charges would exert at a point in space. It is a vector pointing away from positive source charges and toward negative ones. Field strength from a point...

Entropy (S) measures how many microscopic arrangements are available to a system, often described as disorder or energy dispersal. In a spontaneous process the entropy of the universe increases, but the system's own entropy can fall if t...

Culture is the shared beliefs, values, norms, language, symbols and objects of a group, learned through socialization rather than inherited. The MCAT splits it into material culture (objects) and symbolic culture (ideas, norms, language)...

The lymphatic system is a one-way network of vessels, nodes, and organs that returns leftover interstitial fluid (lymph) to the blood, carries dietary fat absorbed by intestinal lacteals, and houses lymphocytes. Testable point: blocked l...

CO₂ is carbon dioxide, the fully oxidized carbon end product of fuel breakdown, released in decarboxylation steps such as pyruvate dehydrogenase and the citric acid cycle. In blood it travels mostly as bicarbonate, so a rise in CO₂ lower...

An acid is a species that donates a proton (Brønsted-Lowry), raises H⁺ in water (Arrhenius), or accepts an electron pair (Lewis). The MCAT most often uses Brønsted-Lowry: a stronger acid has a larger Ka, a lower pKa and a more stable con...

Intelligence is the cognitive capacity to learn from experience, reason, solve problems, and adapt. Theories differ: Spearman's single general factor (g) versus multiple types, as in Gardner's multiple intelligences and Sternberg's triar...

An amide is a carboxylic acid derivative, RC(=O)NR′₂, with nitrogen bonded directly to the carbonyl carbon. Resonance between the nitrogen lone pair and the C=O makes the C–N bond partially double, so amides are planar, non-basic, and th...

Status is a socially defined position in a group or society, with expected rights, duties and a level of prestige. The MCAT mostly tests the types: ascribed (assigned, not chosen), achieved (earned or chosen), and master status (the one...

Power is the rate at which work is done or energy is transferred, measured in watts (1 W = 1 J/s). The most testable point is that power depends on time as well as work: the same work done faster takes more power.

Osmotic pressure is the pressure that must be applied to a solution to stop net flow of solvent into it across a semipermeable membrane. It is colligative (Π = iMRT), so it depends on particle concentration, not identity. Water flows tow...

Motivation is the set of internal and external factors that start, direct, and sustain goal-directed behavior. The MCAT tests which theory explains a behavior (instinct, drive reduction, arousal, incentive, Maslow's hierarchy) and intrin...

Membrane potential is the voltage across a cell membrane (inside relative to outside), about −70 mV in a resting neuron. It comes from ion gradients plus selective permeability, mainly K⁺ leak channels. Testable: it shifts toward the equ...

A glycosidic bond is the covalent C–O–C link, formed by condensation, between a sugar's anomeric carbon and an oxygen of another molecule, usually another sugar's OH. Its α or β configuration matters: amylase cleaves α-1,4 links but not...

SN1 (substitution, nucleophilic, unimolecular) is a two-step reaction: the leaving group departs to form a carbocation, then the nucleophile attacks. The rate depends only on the substrate, and tertiary, allylic and benzylic substrates t...

Self-concept is the set of beliefs a person holds about who they are, built from self-schemas, personal traits, social roles and group identities. It is shaped by social interaction and feedback from others, and it is distinct from self-...

Intensity is the power a wave delivers per unit area perpendicular to its travel, in W/m². It is proportional to amplitude squared, and for a point source it falls as 1/r², so doubling the distance cuts intensity to one quarter.

pH is the negative base-10 logarithm of the hydrogen ion concentration: pH = −log[H⁺]. Lower pH means more acidic, and each pH unit is a tenfold change in [H⁺]. At 25 °C, pH 7 is neutral.

Depression (major depressive disorder) is a mood disorder with depressed mood or loss of interest and pleasure lasting at least two weeks, plus changes in sleep, appetite, energy and thinking. Most testable: it is unipolar, with no manic...