1,800+ MCAT terms, defined.

A conjugated system has p orbitals on adjacent atoms overlapping in a continuous chain, usually alternating single and double bonds, so π electrons are delocalized. The most testable point: conjugation stabilizes a molecule, and longer c...

Gene expression is the use of a gene's DNA sequence to make a functional product, usually a protein, by transcription then translation (some genes give a functional RNA only). Most testable: regulation is often greatest at transcription...

Intellectualization is a defense mechanism in which a person handles a threatening or painful situation by focusing on abstract facts and logic, so the feelings are avoided. The testable cue is cool, detached, analytical talk about a dis...

Gravitational force is the attractive force between any two objects that have mass, directed along the line joining their centers. Its magnitude is proportional to the product of the masses and inversely proportional to the square of the...

Fick's law says the net flux of a solute by diffusion is proportional to the concentration gradient and points from high to low concentration. Testable point: doubling the gradient doubles the flux, and a larger diffusion coefficient D (...

Socialism is an economic and political system in which the means of production are owned collectively or by the state, aiming at a more equal distribution of resources. The most testable point is the contrast with capitalism, where priva...

Dominance is the allele relationship in which the dominant allele's phenotype appears in a heterozygote and masks the recessive allele's. In complete dominance, Aa looks like AA, so Aa × Aa gives a 3:1 phenotype ratio. Dominant does not...

Eclipsed describes a conformation about a C–C single bond in which substituents on adjacent carbons line up (dihedral angle 0° in a Newman projection). It is a high-energy form because of torsional strain, plus steric strain for bulky gr...

Disgust is a basic emotion of revulsion toward something offensive or contaminating. Ekman counted it among six basic emotions (with happiness, sadness, anger, fear, surprise) whose facial expressions are widely recognized across culture...

Noncompetitive inhibition: an inhibitor binds an allosteric site on the free enzyme or the enzyme–substrate complex, so less enzyme is active. In the pure case Vmax decreases and Km stays the same, and extra substrate cannot reverse it....

Static friction is the force parallel to the contact surface that keeps two surfaces from sliding past each other while they are at rest relative to each other. It adjusts to match the applied force up to a maximum, fs ≤ μsN. Equality ho...

A coordinate covalent bond is a covalent bond in which both shared electrons come from one atom, the donor (a Lewis base), and go to an electron-poor acceptor (a Lewis acid). Once formed, it is identical to any other covalent bond of tha...

A psychological disorder is a persistent pattern of thoughts, emotions, or behaviors that is deviant, distressing, and dysfunctional, and in some cases dangerous. The testable point is that being unusual is not enough; distress or impair...

A mirror image is the reflection of a molecule. If the reflection cannot be superimposed on the original, the molecule is chiral and the pair are enantiomers. Most testable: enantiomers have opposite configuration at every stereocenter,...

Broca's area is a region of the left frontal lobe in most people that controls the production of speech. Damage causes Broca's (expressive, nonfluent) aphasia: effortful, halting speech with comprehension largely intact. The MCAT contras...

Homologous chromosomes are the two members of a pair, one from each parent, with the same genes at the same loci but possibly different alleles. They pair in prophase I and separate in anaphase I of meiosis. In evolution, homologous stru...

A concave mirror curves inward like the inside of a spoon and converges light to a focal point at f = r/2, so f is positive. Beyond f its image is real and inverted; inside f it is virtual, upright and enlarged.

The empirical formula gives the simplest whole-number ratio of atoms of each element in a compound. A molecular formula is a whole-number multiple of it, so different compounds can share one (C₂H₂ and C₆H₆ are both CH). Finding it from m...

DNA is a double helix of two antiparallel strands, each a sugar-phosphate backbone carrying the bases A, T, G and C. A pairs with T and G with C by hydrogen bonds, so the strands are complementary and, in double-stranded DNA, A = T and G...

The auditory cortex is the region of the temporal lobe that receives and interprets sound information relayed from the thalamus (medial geniculate nucleus). Each ear projects to both hemispheres, so damage to one side usually does not ca...

Vinylic describes the sp² carbons of a C=C double bond and the atoms (H, halogen) attached directly to them. Most testable: vinylic halides generally resist SN1 and SN2, unlike allylic halides, and vinylic H appear downfield in ¹H NMR.

Uniform circular motion is travel around a circle at constant speed. The velocity still changes direction, so the object accelerates toward the center (a = v²/r), which requires a net inward force. Constant speed does not mean zero accel...

Faraday's constant (F) is the magnitude of the charge carried by one mole of electrons, about 96,500 C/mol (9.65 × 10⁴). It converts between coulombs and moles of electrons, so it links current and time to the amount of product in electr...

A meritocracy is a system in which position and rewards are assigned by merit (ability, effort, achievement) rather than ascribed traits like family background. It is an ideal; conflict theorists argue unequal access to resources means r...

The sclera is the tough, white, opaque outer layer of the eyeball. It protects the eye, maintains its shape, and anchors the eye muscles. It is continuous with the clear cornea at the front, and light enters through the cornea, not the s...

Metabolic rate is the energy an organism uses per unit time, often measured by O₂ consumption or heat output. Per gram of tissue, smaller animals usually have higher rates than larger ones, and endotherms have higher rates than ectotherm...

Glycogen synthesis (glycogenesis) builds the branched glucose-storage polymer glycogen, mainly in liver and muscle, using UDP-glucose as the activated donor. Glycogen synthase adds glucose by α-1,4 bonds, and insulin promotes the process...

Acid-base separation is a liquid-liquid extraction that moves acidic or basic compounds from an organic solvent into water by converting them to ionic salts, leaving neutral compounds behind. Testable point: NaHCO₃ extracts carboxylic ac...

The laws of thermodynamics describe how energy and entropy behave. The first says energy is conserved: ΔU = Q − W. The second says the total entropy of a system plus its surroundings increases in a spontaneous process, so heat flows spon...

A conjugate acid is the species formed when a Brønsted-Lowry base accepts a proton, so it has one more H⁺ than the base (NH₃ → NH₄⁺). The stronger the base, the weaker its conjugate acid, and for a conjugate pair Ka × Kb = Kw.

Self-image is a person's description of their own traits, abilities, and roles. Many texts treat it as one component of self-concept, and some use the two terms interchangeably. Testable point: self-image describes, while self-esteem eva...

Fluorescence is the emission of a photon when an excited electron drops back to the ground state within nanoseconds of absorbing light. The emitted light typically has lower energy and a longer wavelength than the absorbed light, because...

A pure crystalline solid usually melts over a narrow range (about 1–2 °C) near its literature value. Impurities typically lower the melting point and broaden the range, as in freezing point depression. A mixed melting point with an authe...

Hydrophobic interactions are the tendency of nonpolar groups to cluster together in water, which releases ordered water molecules and increases the entropy of the water. They help drive protein folding, burying nonpolar side chains in th...

Brain waves are rhythmic electrical patterns from large groups of cortical neurons, recorded by EEG and classified by frequency. Wakefulness shows fast, low-amplitude waves (beta, then alpha when relaxed). Deeper sleep brings slower, hig...

Suppressor T cells, now usually called regulatory T cells (Tregs), are T cells that turn down the activity of other T and B cells after a threat is cleared and help prevent attack on self. Testable point: they dampen the immune response,...

The endocrine system is a network of glands that release hormones into the blood to act on distant target cells that have matching receptors. Compared with the nervous system, its effects are usually slower but longer-lasting, and the hy...

A resultant wave is the single wave formed when two or more waves overlap in the same medium. At each point its displacement is the sum of the individual displacements (superposition). In-phase waves give a larger amplitude, and waves 18...

The change in Gibbs free energy (ΔG) tells whether a process is thermodynamically favorable at constant temperature and pressure. ΔG < 0 is spontaneous (exergonic), ΔG > 0 is nonspontaneous, and ΔG = 0 is equilibrium. Spontaneous says no...

Recrystallization (recrystallisation) purifies a solid by dissolving it in a minimum of hot solvent and cooling, so the compound crystallizes while most impurities stay dissolved. Most testable: the solvent should dissolve the compound w...

The somatosensory cortex is the strip of parietal lobe along the postcentral gyrus that processes touch, pressure, temperature, pain, and body position. Its body map gives more cortex to sensitive areas like the lips and fingers, not to...

Ketone body formation (ketogenesis) is the liver's mitochondrial conversion of acetyl-CoA, mainly from fatty acid β-oxidation, into acetoacetate and β-hydroxybutyrate, plus a little acetone. It rises in fasting or low insulin, and the br...

Coulomb's constant k ≈ 9.0 × 10⁹ N·m²/C² is the proportionality constant in F = kq₁q₂/r², and k = 1/(4πε₀). Its huge size means small charges exert large forces. In a dielectric medium the force is reduced by the dielectric constant κ.

sp3d is the hybridization of a central atom with five electron domains, giving a trigonal bipyramidal electron geometry. It explains expanded octets in period 3 or lower, as in PCl₅. The textbook model uses one d orbital, though some acc...

A variable-ratio schedule is an operant conditioning schedule that reinforces a response after an unpredictable number of responses, averaging around a set value (slot machines are the classic example). It typically produces a high, stea...

Receptor-mediated endocytosis is a selective way for a cell to take in specific molecules: a ligand binds membrane receptors, usually in clathrin-coated pits, and the membrane pinches inward as a vesicle. It is energy-requiring and satur...

In organic chemistry, oxidation of a carbon means gaining bonds to oxygen (or another more electronegative atom) or losing bonds to H; reduction is the reverse. Most testable: alcohol oxidation, where PCC stops at the aldehyde and strong...

Stratification is the ranking of people into layers of a social hierarchy based on unequal access to wealth, power and prestige. Functionalists explain it as a way to reward important roles; conflict theorists explain it as powerful grou...

"Observer's" is a fragment, not a full MCAT term. The exam's usual context is the observer (detector) in the Doppler effect, where the observer's measured frequency differs from the source's when they move relative to each other. Approac...

sp³d² describes a central atom with six electron domains: one s, three p and two d orbitals mix into six equivalent hybrids pointing to the corners of an octahedron (90° apart). In the textbook model, steric number 6 means sp³d², as in S...