1,800+ MCAT terms, defined.

Gate control theory (Melzack and Wall) says pain perception depends on a "gate" in the spinal cord's dorsal horn. Small pain-fiber input tends to open it, while large touch-fiber input and descending brain signals can close it. This is w...

Viroids are infectious agents made of a short, circular, single-stranded RNA with no protein coat. They mainly infect plants and do not typically encode proteins. Key contrast: viroids are naked RNA, prions are infectious protein, and vi...

Reaction order for a reactant is the exponent of its concentration in the experimentally determined rate law; the overall order is the sum of these exponents. It is found from data, such as initial rates, and matches the balanced-equatio...

A dielectric is an insulating material placed between capacitor plates. It polarizes and weakens the electric field inside, so capacitance rises by the dielectric constant κ (C = κC₀). The testable point is whether the battery stays conn...

Tight junctions are belts of proteins (claudins, occludin) that seal neighboring epithelial cells near their apical surface. They restrict paracellular movement, so substances mostly cross through the cells instead of between them. The s...

A social group is two or more people who interact regularly and share a sense of belonging or common identity. The MCAT mainly contrasts primary groups (small, close, lasting) with secondary groups (larger, impersonal, goal-oriented), an...

Conformational isomers (conformers) are arrangements of one molecule that differ only by rotation about single bonds, so they interconvert without breaking any bond. Most testable: staggered forms are more stable than eclipsed, and in bu...

Calorimetry is the measurement of heat transferred in a process by tracking the temperature change of a known mass of material, usually water, in an insulated container. Its key point is that, ideally, heat released by the system equals...

The normal force is the contact force a surface exerts on an object, directed perpendicular to the surface and pushing the object away from it. Its size depends on the other forces and on any acceleration perpendicular to the surface. It...

Dramaturgy is Erving Goffman's approach that treats everyday social interaction as a theatrical performance, in which people manage the impressions others form of them. Most testable: behavior differs between the front stage (performing...

Germ layers are the three embryonic tissue layers (ectoderm, mesoderm, endoderm) formed at gastrulation. Most testable: nervous system and epidermis come from ectoderm, muscle, bone and blood from mesoderm, and gut and respiratory lining...

Heat capacity is the heat a sample must absorb to raise its temperature by 1 K (or 1 °C), C = q/ΔT. A larger value means a smaller temperature change for the same heat. It depends on sample size; specific heat is the per-mass version.

The critical angle is the angle of incidence, in the medium with the higher index of refraction, at which the refracted ray travels along the boundary (refraction angle 90°). At larger incident angles, light undergoes total internal refl...

Max Weber was a German sociologist known for rationalization and bureaucracy, three types of authority (traditional, charismatic, legal-rational), and a stratification model of class, status and power. The most testable point is that cla...

A protic solvent has a hydrogen on O or N (O–H, N–H), so it can donate hydrogen bonds; water and alcohols are examples. Most testable: polar protic solvents stabilize anions and carbocations, tending to favor SN1 and slow SN2 by weakenin...

Addiction is compulsive seeking and use of a substance despite harmful consequences. The MCAT ties it to the brain's dopamine reward pathway (VTA to nucleus accumbens) and to reinforcement: the drug's effect or relief of withdrawal stren...

The atomic number (Z) is the number of protons in an atom's nucleus, and it defines the element. A neutral atom has Z electrons. The most testable point is that isotopes share Z but differ in neutron number, and only nuclear reactions ch...

Laminar flow is smooth, orderly fluid flow in parallel layers that do not mix, favored by low speed, small radius and high viscosity. Poiseuille's law applies to it. Flow becomes turbulent when the Reynolds number is high, above roughly...

Communication is the transfer of information from a sender to a receiver by verbal (language) and nonverbal means (expressions, gestures, eye contact, personal space). Testable point: basic facial expressions are widely recognized across...

Evolution is a change in the allele frequencies of a population over generations. Natural selection is one cause, alongside genetic drift, mutation, gene flow and nonrandom mating. Populations evolve; individuals do not.

A lyase is an enzyme (EC class 4) that breaks C–C, C–O, or C–N bonds without water and without redox, typically leaving a double bond. It can also run in reverse, adding a group across a double bond. On the MCAT, tell it apart from hydro...

The α-carbon is the carbon directly bonded to a carbonyl carbon. Its hydrogens are weakly acidic (pKa near 20 for a simple ketone) because losing H⁺ gives a resonance-stabilized enolate, so the α-carbon is usually the nucleophilic site i...

Decision making is the cognitive process of choosing among alternatives. The MCAT contrasts slow, deliberate reasoning (weighing options, algorithms) with fast heuristics, which are mental shortcuts that are efficient but can produce sys...

Turbulent flow is chaotic fluid motion with eddies and swirls, instead of smooth parallel layers. It becomes likely at a high Reynolds number (Re = ρvD/η, above roughly 3000 in a pipe) and wastes more energy, so laminar-flow laws such as...

The demographic transition is the shift of a society from high birth and death rates to low birth and death rates as it industrializes. Death rates usually fall before birth rates, so population growth is fastest in the middle stage, whe...

A free radical is an atom or molecule with an unpaired electron, usually made by homolytic bond cleavage from heat or light, and it is highly reactive. Radical stability runs 3° > 2° > 1° > methyl, so radical halogenation favors the more...

Dehydration (a condensation reaction) joins two molecules by removing the elements of water, so H₂O is released. It forms peptide, glycosidic and ester bonds. Linking n monomers into a linear chain releases n − 1 waters, and the reverse...

Gentrification is the movement of higher-income residents into a lower-income urban neighborhood, raising rents and property values and often displacing longtime residents. Most testable: it changes the area's class makeup and can deepen...

Codominance is an inheritance pattern in which both alleles are fully expressed in a heterozygote, so both traits show distinctly rather than blending. The classic case is ABO type AB (IAIB). The testable contrast is with incomplete domi...

Freezing point depression is the lowering of a solvent's freezing point when a solute is dissolved in it. It is a colligative property: it depends on the number of dissolved particles (i × molality), not their identity, so 0.1 m CaCl₂ lo...

Electric potential is the electric potential energy per unit charge at a point, V = U/q, measured in volts (J/C). For a point charge V = kQ/r, a scalar that adds algebraically. A positive charge released from rest moves toward lower pote...

In Freud's psychodynamic theory, fixation is arrested psychosexual development: unresolved conflict at a stage keeps some energy tied to it and shapes adult traits, such as oral habits. The testable point is matching a behavior to its st...

Keto names the carbonyl (C=O) form of a compound, most often the keto tautomer that interconverts with an enol (C=C–OH) by moving a proton and shifting a double bond. For simple aldehydes and ketones the keto form is usually strongly fav...

A taste bud is a cluster of gustatory receptor cells, which are chemoreceptors that transduce dissolved chemicals into neural signals. They sit mostly in tongue papillae and detect sweet, sour, salty, bitter and umami. The exam often tes...

Spin is an intrinsic property of the electron, described by the spin quantum number ms, which is either +½ or −½. By the Pauli exclusion principle, two electrons in one orbital must have opposite spins, and atoms with unpaired spins are...

A transverse wave is one in which the oscillation, of the medium's particles or of field vectors, is perpendicular to the direction the wave carries energy. Waves on a string and electromagnetic waves are transverse, while sound in air i...

A post-translational modification is a chemical change made to a protein after the ribosome has made it, such as phosphorylation, glycosylation, or cleavage. It can alter activity, location, or stability without changing the gene. Phosph...

Anchoring bias is the tendency to rely too heavily on the first piece of information (the anchor) when making a judgment, so later estimates stay close to it. The testable point: adjustment away from the anchor is usually insufficient, e...

Binary fission is the asexual reproduction of prokaryotes: the cell replicates its circular chromosome, elongates, and splits into two genetically identical daughter cells. Because it makes clones, variation in bacteria comes mainly from...

The volt (V) is the SI unit of electric potential, potential difference and emf. One volt is one joule per coulomb, which is energy per unit charge. A charge q moving through a potential difference ΔV changes its potential energy by qΔV,...

Decarboxylation is the loss of a carboxyl group as CO₂, leaving a C–H bond in its place. It occurs readily for β-keto acids and malonic acids on gentle heating, through a cyclic six-membered transition state and an enol, while most simpl...

Working memory is the limited-capacity system that holds and manipulates information for seconds while a task is carried out. The MCAT ties it to Baddeley's model (a central executive plus subsystems) and to a capacity of about 7 ± 2 ite...

Branching enzyme is a glycogen-synthesis enzyme that moves a block of about seven glucose residues from the end of an α-1,4 chain to an interior glucose, forming an α-1,6 linkage. Branching makes glycogen more soluble and gives more chai...

Attribution is the process of explaining the causes of behavior, either as dispositional (internal traits, ability, effort) or situational (external circumstances, luck). The most testable point is that observers often overweight disposi...

An endothermic process absorbs heat from its surroundings, so the system's enthalpy rises (ΔH > 0) and the surroundings cool. Most testable point: raising the temperature shifts an endothermic equilibrium toward products, and K increases.

Electric potential energy (U) is the energy a system of charges stores because of their positions. For two point charges, U = kq₁q₂/r, positive for like charges and negative for opposite charges, taking U = 0 at infinite separation. The...

Top-down processing is perception guided by prior knowledge, expectations, and context, which shape how sensory input is interpreted. It contrasts with bottom-up processing, which builds a percept from the stimulus features. Expect it to...

Centrioles are short cylinders of nine microtubule triplets, found in pairs inside the centrosome of animal cells. The centrosome organizes the spindle during cell division, and centrioles also form the basal bodies of cilia and flagella...

Cyclohexane is a six-carbon ring of sp³ carbons, C₆H₁₂. It is puckered, not flat, and its chair conformation has almost no angle or torsional strain, so the chair is the most stable form. Bulky substituents generally prefer equatorial po...

The pons is a hindbrain and brainstem structure between the medulla and midbrain. It relays signals between the cerebrum and cerebellum and helps regulate sleep and arousal, including REM sleep. Most testable: it belongs to the hindbrain...