Glossary
HESI A2 glossary
The words these questions assume you already know, in plain language. Each term lists the guides that teach it, and each guide links to the term where it first appears.
- acidAn acid is a substance that gives up hydrogen ions in water, which lowers pH below 7 and lets it neutralize a base.
- action potentialAn action potential is the all-or-nothing reversal of a neuron's membrane voltage that carries a signal along its axon.
- acuteAcute describes a condition that begins quickly and is expected to be short-lived, which is a statement about time and not about severity.
- afebrileAfebrile means without fever, and the word is built from the negation prefix a- plus febrile, meaning feverish.
- alleleAn allele is one version of a gene; a person carries two for most genes, one on each chromosome of a homologous pair, and the pair is the genotype.
- alveolusAn alveolus is one of the thin-walled air sacs at the end of the airway, where oxygen and carbon dioxide move between the air and the blood.
- ambulatoryAmbulatory means able to walk, or a care setting that does not require an overnight stay.
- antecedentAn antecedent is the noun a pronoun stands in for; if a reader cannot point back to exactly one noun, the pronoun has failed regardless of its grammar.
- antibodyAn antibody is a protein made by plasma cells, derived from activated B cells, that binds one specific antigen and marks it for destruction.
- atomic numberAn element's atomic number is the number of protons in its nucleus, and it is what decides which element the atom is.
- ATPATP is the molecule cells use to carry energy — breaking off its third phosphate releases the energy a reaction needs.
- author purposeAuthor purpose is the reason a passage was written — to inform, to explain, to persuade, to entertain or to describe.
- benignBenign describes something that is not cancerous and not harmful in itself, or a condition that runs a mild course.
- bolusA bolus is a single dose given quickly and in full, as opposed to the same amount delivered slowly over time; the word also names a mass of chewed food ready to be swallowed.
- bufferA buffer is a solution that resists a change in pH when acid or base is added, usually by pairing a weak acid with its partner base.
- cause and effectA cause-and-effect structure explains why something happened, linking an event to the conditions that produced it.
- cell theoryThe cell theory is the claim that every living thing is made of cells, that the cell is the basic unit of structure and function, and that cells come from existing cells.
- chain of infectionThe chain of infection is the six links an infection needs — agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host — and breaking any one of them prevents it.
- chemical changeA chemical change makes a new substance with new properties — burning, rusting and digesting food are chemical changes.
- chromosomeA chromosome is one long DNA molecule wound around histone proteins, and a human body cell carries 46 of them, in 23 pairs.
- chronicChronic describes a condition that persists over a long period or recurs, as distinct from one that is severe.
- clauseA clause is a group of words with its own subject and verb; an independent clause can stand as a sentence, a dependent one cannot.
- cohesionCohesion is the attraction between water molecules that lets water hold together — the property behind surface tension and the continuous water column in a plant.
- combining vowelA combining vowel — almost always o — is the vowel placed between a root and a suffix when the suffix starts with a consonant.
- comma spliceA comma splice joins two independent clauses with only a comma; a comma is not strong enough to hold two sentences together, so one of four repairs is needed.
- compare and contrastA compare-and-contrast structure sets two things side by side, showing how they are alike and how they differ, usually along the same points.
- confusable wordConfusable words are pairs that look or sound alike but differ in meaning and grammar, such as affect and effect, or accept and except.
- context clueA context clue is information in the sentence around an unknown word that narrows what the word means — a definition, a restatement, a contrast, an example, a consequence or the general sense.
- contraindicationA contraindication is a condition or circumstance in which a treatment should not be used, because its risk outweighs its benefit.
- covalent bondA covalent bond is a shared pair of electrons between two atoms, the usual bond between nonmetals and the bond that holds molecules together.
- cross multiplicationCross multiplication solves a proportion by multiplying each numerator by the opposite denominator, then dividing by the remaining number.
- dangling modifierA dangling modifier is a descriptive phrase left with nothing in the sentence to describe — its intended subject is missing, so the sentence says something absurd.
- decimalA decimal writes a fraction using place value after a decimal point, so 0.75 means 75 hundredths, and the alignment of that point governs every operation.
- denominatorThe denominator is the bottom number of a fraction: how many equal parts the whole is divided into, and therefore the size of each part.
- dependent clauseA dependent clause has a subject and a verb but opens with a word that leaves it unfinished, so it cannot stand alone as a sentence.
- dependent variableThe dependent variable is what an experiment measures — the outcome that changes in response to the independent variable.
- dermisThe dermis is the layer under the epidermis: connective tissue carrying the blood vessels, nerves, glands and hair follicles the skin needs.
- diaphysisThe diaphysis is the long shaft of a long bone — a tube of compact bone wrapped around a marrow cavity.
- direct objectA direct object is the noun or pronoun that receives an action verb — the answer to "what?" or "whom?" after the verb.
- dominantA dominant allele is one whose trait appears when at least one copy is present, masking the recessive allele paired with it.
- drop factorA drop factor is the number of drops an intravenous set delivers per milliliter, printed on the set's packaging, and it is what turns a volume into a drop count.
- effectorAn effector is what carries out a feedback loop's response — a muscle that contracts or a gland that secretes.
- epidermisThe epidermis is the outer, avascular layer of the skin — stratified epithelium that renews from its base and makes keratin and melanin.
- epiphysisThe epiphysis is the rounded end of a long bone — spongy bone under a thin shell, shaped to meet the joint it forms.
- epithelial tissueEpithelial tissue covers and lines surfaces, forms glands, and sits on a basement membrane — it has no blood vessels of its own.
- equilibriumEquilibrium is the state of a reversible reaction in which the forward and reverse rates are equal, so the amounts stop changing while both directions still run.
- eukaryoteA eukaryote is a cell with a true nucleus and membrane-bound organelles — the cells of plants, animals, fungi and protists.
- euphemismA euphemism is a mild or indirect word used in place of one that is harsh or unwelcome, such as "passed away" for "died".
- exocytosisExocytosis is a vesicle fusing with the cell membrane and releasing its contents outside the cell — how a cell secretes what it makes.
- exponentAn exponent states how many times a base is multiplied by itself, so 10³ means 10 × 10 × 10, and for a power of ten, each step moves the decimal point one place.
- fact and opinionA fact can be checked and shown true or false; an opinion is a judgment, preference or prediction that a reader could reasonably dispute.
- flexionFlexion is a movement that decreases the angle at a joint, as bending the elbow brings the hand toward the shoulder.
- geneA gene is a section of DNA that carries the instructions for a trait or a protein, and the alternative versions of a gene are its alleles.
- genotypeA genotype is the pair of alleles an organism carries for a gene, written as letters such as BB, Bb or bb.
- glycolysisGlycolysis is the first stage of cellular respiration: in the cytoplasm, one glucose is split into two pyruvate, with a small net gain of ATP.
- Gram stainA Gram stain is a four-reagent differential stain that sorts bacteria by cell wall: a thick wall holds the purple dye (Gram-positive), a thin wall loses it and takes the pink counterstain (Gram-negative).
- half-lifeA half-life is the time it takes for half of the atoms in a sample to decay; the remaining amount halves again every half-life and never reaches zero.
- homeostasisHomeostasis is the maintenance of a stable internal environment while conditions outside the body change — a loop that keeps correcting, not a fixed state.
- hormoneA hormone is a chemical messenger released into the blood that changes the activity only of cells that carry a receptor for it.
- hydrogen bondA hydrogen bond is a weak attraction between a hydrogen already bonded to oxygen, nitrogen or fluorine and a nearby lone pair on another molecule, or on another part of the same molecule.
- hypodermisThe hypodermis is the fatty layer under the dermis — subcutaneous tissue that insulates, cushions, stores energy and anchors the skin.
- hypothesisA hypothesis is a testable, falsifiable statement that predicts what an experiment will show — the proposed answer a controlled test is designed to check.
- independent clauseAn independent clause has its own subject and a finite verb and can stand alone as a sentence; a dependent clause has the same parts and still cannot.
- independent variableThe independent variable is the one condition an experimenter changes on purpose — the input a hypothesis varies to see what follows.
- indirect objectAn indirect object is the noun or pronoun that receives the direct object — the "to whom?" or "for whom?" — and it appears only when a direct object is already there.
- inferenceAn inference is a conclusion the passage supports without stating — one step past the text, using only what the text gives.
- ionAn ion is an atom or group of atoms with a charge, made by losing electrons (positive cation) or gaining them (negative anion).
- ionic bondAn ionic bond is the attraction between two oppositely charged ions, formed when one atom gives electrons to another — a metal and a nonmetal, usually.
- isotopeIsotopes are atoms of the same element with different numbers of neutrons, so they share chemistry but differ in mass.
- IV rateAn IV rate is the volume an infusion delivers per hour, found by dividing the total volume by the number of hours it runs.
- kelvinThe kelvin is the SI unit of temperature: its zero is absolute zero, its step is the size of a Celsius degree, and kelvin = °C + 273.15.
- Krebs cycleThe Krebs cycle is the second stage of cellular respiration, in the mitochondrial matrix, where acetyl-CoA is broken down and most of the electron carriers are loaded.
- least common denominatorThe least common denominator is the smallest number that both denominators divide into evenly, and it is the shared bottom number used to add or subtract fractions.
- main ideaThe main idea is the claim a passage was written to deliver — broad enough to cover every paragraph, narrow enough that no paragraph sits outside it.
- matterMatter is anything with mass that takes up space, and it exists as a solid, a liquid, a gas or a plasma.
- meiosisMeiosis is the division that makes gametes: two divisions after one DNA replication turn one diploid cell into four haploid cells.
- metric prefixA metric prefix names a power of ten applied to a base unit — kilo is 1,000, centi is one hundredth, milli is one thousandth — so converting between them moves the decimal point.
- midsagittal planeThe midsagittal plane is the vertical plane down the body's midline, dividing it into equal left and right halves.
- mitosisMitosis is the division that grows and repairs a body: one cell becomes two, and each daughter cell carries the same set of chromosomes as the parent.
- mixed numberA mixed number combines a whole number and a proper fraction, such as 1 1/2, and is converted to an improper fraction before multiplying or dividing.
- modifierA modifier is a word or phrase that describes another word, and it belongs as close as possible to the word it describes.
- molar massMolar mass is the mass of one mole of a substance, in grams per mole, read off the periodic table by adding the atomic masses in the formula.
- molarityMolarity is concentration in moles of solute per liter of solution — not per liter of solvent — and it falls when a solution is diluted, because the moles of solute stay and the volume grows.
- moleA mole is 6.022 × 10²³ particles of a substance — the chemist's counting unit, chosen so that one mole of a compound weighs its formula mass in grams: 18 g of water is one mole of it.
- muscle tissueMuscle tissue shortens to produce movement, and its three kinds — skeletal, smooth and cardiac — differ in where they are and whether they obey you.
- myelin sheathThe myelin sheath is fatty insulation wrapped around some axons that speeds the impulse by letting it jump between the nodes of Ranvier.
- negative feedbackNegative feedback is a loop in which the response reverses the change that triggered it, bringing the value back toward its set point.
- nephronA nephron is the functional unit of the kidney: a filtering corpuscle followed by the tubule that turns the filtrate into urine.
- nervous tissueNervous tissue carries electrical signals: neurons generate and conduct them, and glial cells insulate, feed and support the neurons.
- neuronA neuron is a nerve cell that receives, carries and passes on information as an electrical impulse, and its working parts are the dendrites, the cell body and the axon.
- nucleotideA nucleotide is the building block of DNA and RNA: a phosphate, a five-carbon sugar, and one of the nitrogenous bases.
- numeratorThe numerator is the top number of a fraction: how many of the equal parts named by the denominator are being counted.
- order of operationsThe order of operations is the fixed sequence for evaluating an expression: parentheses, exponents, multiplication and division left to right, then addition and subtraction left to right.
- osmosisOsmosis is the movement of water across a selectively permeable membrane toward the side with more dissolved material, and it needs no energy.
- osteoblastAn osteoblast is the cell that builds bone: it lays down the new matrix and starts the mineral deposit that hardens it.
- osteoclastAn osteoclast is the large, multinucleated cell that breaks bone down, releasing calcium into the blood and reshaping the skeleton.
- osteocyteAn osteocyte is a mature bone cell, once an osteoblast, sealed inside the matrix it made — it maintains that matrix and senses load.
- oxidationOxidation is the loss of electrons by a species; it always comes with a reduction, in which another species gains them.
- parallel structureParallel structure puts items in a list or a comparison in the same grammatical shape — all nouns, all verbs, or all clauses — so the reader can see the pattern.
- part of speechA part of speech is the job a word is doing in a particular sentence — noun, pronoun, verb, adjective, adverb, preposition, conjunction or interjection — not a label the word carries with it.
- percent changePercent change measures a change against the starting value: the amount of change divided by the original, then written as a percent.
- pericardiumThe pericardium is the sac around the heart — a fibrous outer layer and a fluid-filled inner space that lets it beat without friction.
- periosteumThe periosteum is the tough membrane wrapped around a bone's outer surface, carrying its blood vessels and the cells that widen and mend it.
- pHpH is a measure of how acidic a solution is — how many hydrogen ions it holds — on a scale that usually runs from 0 to 14, with 7 neutral.
- phagocytosisPhagocytosis is a cell taking in a solid particle by wrapping its membrane around it, then digesting it in a vesicle.
- phenotypeA phenotype is the trait an organism actually shows, so two different genotypes can produce the same one.
- phospholipid bilayerThe phospholipid bilayer is the two-layer sheet that makes up every cell membrane, with water-loving heads facing out and water-fearing tails facing in.
- phraseA phrase is a group of related words that works as one part of speech and has no subject-verb pair of its own.
- physical changeA physical change alters a substance's form, size or state without making a new substance — melting, cutting and dissolving are the usual examples.
- pleuraThe pleura is the double membrane around each lung — one layer on the lung, one on the chest wall, with a thin film of fluid between them.
- position prefixA position prefix says where something is — pre- before, post- after, sub- under, supra- above, peri- around, inter- between.
- positive feedbackPositive feedback is a loop in which the response amplifies the change that triggered it, so a process runs to a finish instead of settling.
- predicate adjectiveA predicate adjective is an adjective after a linking verb that describes the subject, as in "the dressing was loose", and it takes no object.
- predicate nominativeA predicate nominative is a noun or pronoun after a linking verb that renames the subject, as in "Ms. Okafor is the charge nurse".
- prefixA prefix is the part fixed to the front of a medical word, and it changes direction, size, speed, number or negation without changing what the word is about.
- problem and solutionA problem-and-solution structure states a problem and then presents one or more fixes, often weighing their limits.
- prokaryoteA prokaryote is a cell with no nucleus and no membrane-bound organelles — its DNA sits loose in the cytoplasm, as it does in bacteria.
- pronoun casePronoun case is the form a pronoun takes for the job it does — subject (I, he, she), object (me, him, her) or possessive (my, his, her).
- proportionA proportion is a statement that two ratios are equal, with one of the four values missing — and the missing value is what the calculation returns.
- proximalProximal means nearer the trunk, or nearer the point where a limb attaches — the opposite of distal.
- ratioA ratio compares two quantities, written a : b, and it stays the same when both sides are multiplied or divided by the same number.
- receptorA receptor is the structure that detects a change — the sensor of a feedback loop, reporting temperature, pressure, chemistry or stretch.
- recessiveA recessive allele is one whose trait appears only when both copies are recessive, because a dominant partner masks it.
- reconstitutionReconstitution is dissolving a powdered medication with a stated volume of diluent so the vial holds a known concentration, which then feeds the proportion.
- reflex arcA reflex arc is the pathway a reflex travels — receptor, sensory neuron, a synapse in the CNS, motor neuron and effector — without waiting for the brain.
- rootThe root is the part of a medical word that says what the word is about — the organ, the tissue or the substance — and everything else in the word modifies it.
- roundingRounding replaces a number with a near one at a chosen place value, using the digit after it: five or more rounds up, less than five rounds down.
- run-on sentenceA run-on sentence joins two independent clauses with no punctuation at all, while a comma splice joins them with only a comma.
- sarcomereA sarcomere is one repeating contractile unit of a muscle fiber, running from one Z line to the next, and it is the part that shortens in a contraction.
- selectively permeableA selectively permeable membrane lets some substances cross freely while blocking others, which is what makes a cell a separate compartment.
- semicolonA semicolon joins two independent clauses without a conjunction, and separates list items that already contain commas.
- sentence fragmentA sentence fragment is a group of words punctuated as a sentence that lacks a subject, a verb or a complete thought.
- sequenceA sequence structure presents information in time order, using words like first, then, after and finally as its skeleton.
- signal wordA signal word is a transition that tells the reader what the next sentence is doing — adding, contrasting, concluding or giving a cause.
- sodium-potassium pumpThe sodium-potassium pump is a membrane protein that spends ATP to push three sodium ions out and pull two potassium ions in.
- somatic nervous systemThe somatic nervous system is the motor division that carries commands to skeletal muscle, the muscle a person moves on purpose.
- specific heatSpecific heat is the energy needed to raise one gram of a substance by one degree Celsius, in J/(g·°C); water's 4.18 is high, so it resists a temperature change.
- subject-verb agreementSubject-verb agreement means the verb matches its subject in number — a singular subject takes a singular verb, a plural subject a plural verb.
- suffixA suffix is the part fixed to the end of a medical word, and it says what is being done or what kind of word this is — a condition, an inflammation, a procedure, a specialist.
- summaryA summary states the main idea and the few details needed to support it, in your own words and much shorter than the original.
- supporting detailA supporting detail is a fact, example or reason that holds up the main idea of a passage rather than adding a new point of its own.
- synapseA synapse is the junction where a neuron passes its signal to another cell, across a narrow gap, by releasing a neurotransmitter.
- text structureText structure is the outline a passage is built on — sequence, cause and effect, compare and contrast, or problem and solution — and naming it tells you where the claim sits.
- tissueA tissue is a group of similar cells with a shared job and the material between them — the level between a cell and an organ.
- toneTone is the writer's attitude toward the subject, carried by word choice and sentence shape rather than stated outright.
- unit rateA unit rate is a rate reduced to one unit — dollars per item, kilometers per hour, milligrams per milliliter — which is what makes two offers comparable.
- valence electronA valence electron sits in an atom's outermost occupied shell, and the count there decides which bonds the atom forms and what charge its ion carries.
- verb tenseVerb tense is the form of a verb that places an action in time — past, present or future — and shows whether it is ongoing or finished.
- virusA virus is genetic material wrapped in a protein coat — it has no cell, no metabolism, and can only reproduce inside a host cell.
- word choiceWord choice is the writer's selection of words, which carries tone and shades meaning beyond a word's plain dictionary sense.
- word partA word part is a prefix, root, suffix or combining vowel — one of the pieces a medical word is built from.