1,800+ MCAT terms, defined.

Insomnia is a sleep disorder with persistent difficulty falling asleep, staying asleep, or waking too early, despite adequate opportunity to sleep, and it impairs daytime functioning. The MCAT tests telling it apart from narcolepsy, slee...

A cloning vector is a DNA molecule, usually a bacterial plasmid, that carries a foreign DNA insert into a host cell and replicates there, making many copies. Testable point: antibiotic resistance on the vector shows a cell took up a plas...

McDonaldization is George Ritzer's term for the spread of fast-food restaurant principles into other parts of society. It extends Max Weber's rationalization. The testable point is its four dimensions: efficiency, calculability, predicta...

SOCl₂ is thionyl chloride, a reagent that converts carboxylic acids to acid chlorides and alcohols to alkyl chlorides. The OH becomes a good leaving group, and SO₂ and HCl leave as gases. Testable point: with a chiral alcohol and pyridin...

The refractive index n of a material is the speed of light in vacuum divided by its speed in the material (n = c/v), so n is 1 for vacuum and greater than 1 for ordinary materials. Light entering a higher-n medium slows and bends toward...

The ground state is the lowest-energy state of an atom, with electrons in the lowest available orbitals (hydrogen: n = 1). Absorbing a photon that matches an energy gap lifts an electron to an excited state, and the atom emits that same...

Urbanization is the rise in the share of a population living in cities, driven mainly by rural-to-urban migration and often tied to industrialization. Testable point: it concentrates people and resources, shifting social structure, healt...

The Bowman's capsule is the cup-shaped start of the nephron that surrounds the glomerulus and collects filtrate pushed out of the blood by hydrostatic pressure. Filtration depends on size and charge, so water, ions, glucose and urea pass...

Coupled reactions pair an endergonic reaction (ΔG > 0) with an exergonic one, usually through a shared intermediate, so the overall process can proceed. The testable point: the free energy changes add, and the pair is spontaneous only if...

Like charges are charges of the same sign, both positive or both negative, and they repel each other. The force follows Coulomb's law, F = k|q₁q₂|/r², so doubling the separation cuts the force to one-fourth.

Creativity is the ability to produce ideas or solutions that are both novel and useful. The MCAT ties it to divergent thinking, which generates many possible answers, as opposed to convergent thinking, which narrows to one best answer.

In Freud's psychoanalytic theory, the death instinct (often called Thanatos) is an unconscious drive toward aggression, destruction and a return to a lifeless state. It opposes the life instinct, Eros. Aggression or self-destructive beha...

Sound level (intensity level) is a logarithmic measure of a sound's intensity compared with the threshold of hearing, given in decibels (dB). Each tenfold increase in intensity adds 10 dB, and doubling intensity adds about 3 dB.

A combustion reaction is the exothermic reaction of a fuel with O₂; complete burning of a hydrocarbon gives CO₂ and H₂O. The most testable points are balancing it (O last) and that ΔH°comb is negative, while limited O₂ gives CO or soot i...

The phonological loop is the part of Baddeley and Hitch's working memory model that briefly holds verbal and auditory information by silently rehearsing it. It is separate from the visuospatial sketchpad, so two verbal tasks interfere wi...

A checkpoint is a control point in the cell cycle where regulatory proteins check conditions, such as DNA damage or chromosome attachment, and can pause progression until they are met. Testable point: loss of checkpoint proteins like p53...

A disulfide linkage is a covalent S–S bond formed by oxidation of the thiol groups of two cysteine side chains. It stabilizes tertiary or quaternary structure, especially in extracellular proteins, and is broken by reducing agents such a...

Retention time is the time from sample injection to the detector peak of a compound in chromatography (GC or HPLC). The stronger a compound's attraction to the stationary phase, the longer its retention time, so predict elution order fro...

A reinforcement schedule is the rule for when a behavior is reinforced in operant conditioning, set by number of responses (ratio) or elapsed time (interval), and fixed or variable. Most testable: variable-ratio typically gives the highe...

The freezing point is the temperature at which a liquid and its solid coexist in equilibrium at a given pressure; for a pure substance it equals the melting point. While a pure substance freezes, heat leaves but its temperature stays con...

An energy shell is the set of orbitals sharing the same principal quantum number n (1, 2, 3, …). Higher n means higher energy and a larger average distance from the nucleus. A shell holds up to 2n² electrons, and a jump between shells ab...

Universal emotions are basic emotions, commonly listed as happiness, sadness, anger, fear, surprise and disgust, whose facial expressions are recognized across many cultures. The testable point: the expressions are largely shared, but cu...

Absolute configuration is the true three-dimensional arrangement of the four groups on a stereocenter, labeled R or S by Cahn-Ingold-Prelog priority rules. Most testable: the R/S label comes from structure and does not predict whether ro...

Eukaryotic gene regulation is the control of which genes are expressed and how much product is made. Control is often greatest at transcription initiation, where transcription factors act at promoters and enhancers, and chromatin state m...

A conditioned stimulus (CS) is a formerly neutral stimulus that, after repeated pairing with an unconditioned stimulus, comes to trigger a conditioned response. The testable point: the CS is learned and predicts the US, so it must be pre...

The antrum is the lower part of the stomach, just before the pyloric sphincter, where churning turns food into chyme and G cells secrete gastrin. Testable point: antral gastrin stimulates HCl release by parietal cells, and low luminal pH...

The heat of formation (standard enthalpy of formation, ΔH°f) is the enthalpy change when 1 mole of a compound forms from its elements in their standard states. Most testable point: ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants), and element...

Natural frequency is the frequency at which a system oscillates by itself when displaced and released. It is set by the system's own properties, such as mass, stiffness, length or tension. A driving force at this frequency produces reson...

The oval window is the membrane-covered opening between the middle ear and the fluid-filled cochlea. The stapes pushes on it, turning ossicle vibrations into pressure waves in cochlear fluid. Its small area compared with the eardrum conc...

Absorbance (A) measures how much light a sample absorbs at a given wavelength: A = log₁₀(I₀/I) = εbc. It is unitless and, for dilute solutions, directly proportional to concentration and path length. Higher A means less transmitted light.

Umami is the savory taste, the fifth basic taste alongside sweet, sour, salty, and bitter. It is triggered mainly by glutamate, an amino acid found in meats, aged cheese, and MSG. The testable point: taste is a chemical sense, and umami...

SI units are the international standard metric units: meter, kilogram, second, ampere, kelvin, mole and candela. Other units, such as the newton and joule, are combinations of these. Testable point: convert to SI before calculating, espe...

The titrant is the solution of known concentration that is added gradually, usually from a buret, to the analyte of unknown concentration. Its known concentration and measured volume at the equivalence point give the moles of analyte.

Drive-reduction theory proposes that a physiological need (like hunger or thirst) creates an internal state of tension called a drive, and behavior is motivated to reduce it and restore homeostasis. The testable point: it explains motiva...

Fluid balance is the regulation of body water and solute concentration so blood volume, pressure and osmolarity stay in a narrow range, mainly through the kidney. Testable point: high osmolarity raises ADH (more water reabsorbed); low bl...

Electrolysis uses an external power source to drive a nonspontaneous redox reaction in an electrolytic cell. Oxidation still occurs at the anode and reduction at the cathode, but the anode is positive and the cathode negative, the revers...

Doppler ultrasound sends high-frequency sound into the body and measures the frequency shift of echoes from moving reflectors, usually red blood cells, to find their speed. Motion toward the transducer raises the echo frequency, and moti...

To react nucleophilically is to act as a nucleophile: an electron-rich species donates a lone pair or pi bond to form a new bond to an electron-poor atom (the electrophile). Curved arrows start at the electron-rich site and point to the...

Persuasion is changing someone's attitudes or beliefs with a message. In the elaboration likelihood model, the central route relies on careful thought about argument quality and tends to give longer-lasting change; the peripheral route r...

mRNA processing is the set of changes that convert eukaryotic pre-mRNA into mature mRNA in the nucleus: a 5′ cap, splicing out introns, and a 3′ poly-A tail. Most testable: introns are removed and exons joined, and alternative splicing l...

Social influence is the way a person's thoughts, feelings, and behavior change because of the real or imagined presence of others. The MCAT sorts it into conformity (matching a group), compliance (answering a request), and obedience (fol...

An acidic buffer is a solution of a weak acid and its conjugate base (such as acetic acid and acetate) that resists pH change and usually buffers below pH 7. It works best when pH is within about 1 unit of the pKa, and the pH is set by t...

Rotational equilibrium means the net torque on an object is zero, so its angular acceleration is zero: it either does not rotate or spins at constant angular velocity. Most testable point: about a chosen pivot, set clockwise torques equa...

The stress response is the body's physiological and psychological reaction to a stressor, driven by a fast sympathetic pathway (epinephrine) and a slower HPA axis that releases cortisol. Most testable: it follows appraisal of the threat,...

The fundus is the dome-shaped upper part of the stomach, above the esophageal opening. More generally, it is the part of a hollow organ farthest from its opening (the uterus has one too). Testable point: acid-secreting glands lie mainly...

Simple distillation separates liquids by boiling point: the more volatile component vaporizes first, condenses in a condenser, and is collected as distillate. It works well when boiling points differ widely (roughly 25 °C or more); close...

Exchange theory (social exchange theory) says people shape social interactions by weighing perceived rewards against perceived costs, and tend to continue relationships that seem to give a net benefit. It is a micro-level theory, often g...

Hemoglobin binds up to four O₂ cooperatively: each O₂ bound makes the next bind more easily, giving a sigmoidal curve, unlike hyperbolic myoglobin. Most testable: a right shift (lower pH, more CO₂, higher temperature, more 2,3-BPG) means...

Alkali metals are the Group 1 elements other than hydrogen: Li, Na, K, Rb, Cs and Fr. Each has one valence electron, so each forms a +1 cation. Going down the group, ionization energy falls and reactivity rises.

The angle of reflection is the angle between a reflected ray and the normal, the line perpendicular to the surface at the point of contact. By the law of reflection it equals the angle of incidence, and both are measured from the normal,...