Science definitions

66 science terms from Achievable exam prep courses, each defined in plain language with links into the course textbook.

• Acid-base equilibriumAcid-base equilibrium is the reversible balance between an acid and its conjugate base in solution, described by the acid dissociation constant Ka. It determines a solution's pH and how strongly that pH resists change when acid or base is added.
• Amino acidsAmino acids are the building blocks of proteins — molecules containing an amino group, a carboxyl group, and a distinctive side chain attached to a central carbon. Twenty standard amino acids link together in chains to form all the body's proteins.
• Angular momentumAngular momentum is the rotational analogue of linear momentum, describing how much rotational motion an object has. For a rigid body spinning about a fixed axis it equals L = Iω, the moment of inertia times the angular velocity.
• ApoptosisApoptosis is programmed cell death — an energy-dependent, tightly regulated process in which a cell dismantles itself into membrane-bound fragments that neighboring cells clear away. Because the cell's contents are never released, apoptosis does not trigger inflammation.
• Atomic mass vs. atomic numberAtomic number is the number of protons in an atom's nucleus and defines the element, while atomic mass reflects the total of protons and neutrons. Electrons contribute almost nothing to mass — a single electron weighs only about 0.00055 amu.
• Atomic structure and electron configurationElectron configuration is the arrangement of an atom's electrons among its energy levels, subshells, and orbitals, written in notation like 1s² 2s² 2p⁶. It follows from atomic structure and predicts an element's chemical behavior.
• Carbohydrate structureCarbohydrate structure describes how sugars are built: monosaccharides with the general formula Cn(H2O)n link through glycosidic bonds into disaccharides and polysaccharides such as glycogen, starch, and cellulose.
• cDNA cloningcDNA cloning is the technique of copying mature mRNA into complementary DNA with reverse transcriptase and inserting that DNA into a vector for propagation. Because cDNA is built from spliced mRNA, it contains only exons and no introns.
• Cell cycleThe cell cycle is the ordered sequence of events a cell goes through to duplicate its contents and divide into two daughter cells. It consists of interphase — the G1, S, and G2 phases — followed by mitosis and cytokinesis.
• Cell theoryCell theory is the foundational principle of biology stating that all living organisms are made of cells, that the cell is the basic unit of structure and function in life, and that all cells arise from pre-existing cells.
• Chemical kinetics and equilibriumChemical kinetics is the study of how fast reactions proceed, while chemical equilibrium describes the state where forward and reverse reaction rates are equal and concentrations stop changing. Together they explain how quickly a reaction happens and where it ends up.
• Circular motionCircular motion is movement of an object along a circular path. In uniform circular motion the speed is constant, but the velocity's direction constantly changes, requiring a centripetal acceleration a = v²/r directed toward the center.
• Citric acid cycleThe citric acid cycle (Krebs cycle) is a series of mitochondrial reactions that oxidizes acetyl-CoA to carbon dioxide, capturing energy as NADH, FADH₂, and GTP for use in oxidative phosphorylation.
• Concentration cellA concentration cell is an electrochemical cell in which both half-cells contain the same species at different concentrations. It generates voltage purely from that concentration difference, and the current stops once the two sides equalize.
• Conservation of momentumConservation of momentum is the physical law stating that the total momentum of a system stays constant when no net external force acts on it. It is the key principle for analyzing collisions and explosions.
• Consolidation (soil mechanics)Consolidation is the gradual settlement of saturated soil as water is squeezed out of its pore spaces under an applied load. Because water must escape before the soil skeleton can compress, consolidation settlement develops slowly over months or years rather than immediately.
• Covalent bondsA covalent bond is a chemical bond formed when two atoms share one or more pairs of electrons. Covalent bonds hold molecules together and can be single, double, or triple, and polar or nonpolar depending on electronegativity differences.
• DynamicsDynamics is the branch of mechanics that studies bodies in motion and the forces that cause that motion. It combines kinematics, which describes motion, with kinetics, which relates motion to forces through Newton's laws.
• Electromotive forceElectromotive force (emf) is the energy a source such as a battery or generator supplies per unit of charge to drive current around a circuit. Despite its name it is not a force — it is measured in volts, joules per coulomb.
• Electron transport chainThe electron transport chain is a series of protein complexes in the inner mitochondrial membrane that transfers electrons from NADH and FADH2 to oxygen, pumping protons to create the gradient that powers ATP synthesis.
• ElectrostaticsElectrostatics is the branch of physics that studies electric charges at rest — how they attract and repel through Coulomb's law, and the electric fields and potentials they create in the space around them.
• EnzymesEnzymes are biological catalysts, usually proteins, that speed up chemical reactions by lowering their activation energy. Each enzyme binds specific substrates at its active site and is not consumed in the reaction.
• Free body diagramA free body diagram is a sketch that isolates a single object and represents every external force acting on it as a labeled arrow. It is the standard first step for applying Newton's second law to solve force problems.
• Free fallFree fall is motion in which gravity is the only force acting on an object. Near Earth's surface, all objects in free fall accelerate downward at g ≈ 9.8 m/s², regardless of their mass.
• GenotypeA genotype is the specific set of alleles an organism carries at a gene or across its genome. It is the inherited genetic information that, together with environmental influences, produces the organism's observable traits.
• Geometrical opticsGeometrical optics is the branch of optics that models light as rays traveling in straight lines, using the laws of reflection and refraction to explain how mirrors and lenses form images.
• GluconeogenesisGluconeogenesis is the metabolic pathway that synthesizes new glucose from non-carbohydrate precursors such as lactate, glycerol, and glucogenic amino acids, occurring mainly in the liver during fasting.
• Groundwater hydrologyGroundwater hydrology is the study of how water occurs and moves beneath the earth's surface through aquifers — porous layers of soil and rock. It covers the water table, permeability, Darcy's law, and well hydraulics.
• HeterocyclesHeterocycles are ring compounds in which at least one ring atom is an element other than carbon — usually nitrogen, oxygen, or sulfur. They form the core of nucleotide bases, many amino acid side chains, vitamins, and a large share of pharmaceuticals.
• Intermolecular forcesIntermolecular forces are the attractive forces between molecules, as opposed to the bonds within them. From weakest to strongest, the main types are London dispersion forces, dipole-dipole interactions, and hydrogen bonds.
• KinematicsKinematics is the branch of mechanics that describes motion — position, velocity, and acceleration — without considering the forces that cause it. Its core tools are the kinematic equations relating displacement, velocity, acceleration, and time.
• Kinetic and potential energyKinetic energy is the energy of motion, given by KE = ½mv², while potential energy is stored energy due to position or configuration, such as gravitational potential energy PE = mgh. In a system with no friction, the two convert back and forth while their total stays constant.
• Laws of motionThe laws of motion are Newton's three principles describing how forces affect movement: objects resist changes in motion (inertia), net force equals mass times acceleration (F = ma), and every action has an equal and opposite reaction.
• Mechanics of materialsMechanics of materials is the branch of engineering mechanics that studies how solid objects deform and fail under loads, using concepts like stress, strain, and elasticity to analyze members in tension, compression, torsion, and bending.
• Mendelian geneticsMendelian genetics is the set of inheritance principles derived from Gregor Mendel's pea plant experiments, describing how discrete alleles segregate and assort during gamete formation. Its core rules are the laws of segregation, independent assortment, and dominance.
• Molar massMolar mass is the mass of one mole of a substance, expressed in grams per mole. It equals the sum of the atomic masses of every atom in the chemical formula, and it is the conversion factor between the mass of a sample and the number of particles it contains.
• Momentum and impulseMomentum is the product of an object's mass and velocity (p = mv), and impulse is the product of force and the time it acts (J = FΔt). The impulse delivered to an object equals its change in momentum.
• MutarotationMutarotation is the change in optical rotation that occurs when a pure anomer of a cyclic sugar is dissolved and interconverts with its other anomer until equilibrium is reached. The interconversion proceeds through the open-chain form of the sugar.
• Nonenzymatic protein functionNonenzymatic protein function refers to the many jobs proteins perform without catalyzing reactions, including structural support, transport, motion, cell adhesion, signaling, and immune defense. These roles depend on protein shape and binding rather than on catalysis.
• Open channel flowOpen channel flow is the movement of a liquid in a conduit with a free surface exposed to atmospheric pressure, such as a river, canal, or partially filled pipe. Gravity drives the flow rather than a pressure difference.
• OscillationsOscillations are repetitive back-and-forth movements about an equilibrium position, such as a swinging pendulum or a mass bouncing on a spring. They are described by amplitude, period, and frequency.
• Pitot tubeA Pitot tube is an instrument that measures fluid flow velocity by comparing the stagnation pressure at a forward-facing opening with the static pressure of the surrounding flow. It is the standard airspeed sensor on aircraft.
• Plasma membraneThe plasma membrane is the phospholipid bilayer that encloses a cell, separating its interior from the extracellular environment. It is selectively permeable, controlling what enters and leaves while also carrying receptors for cell signaling.
• ProlineProline is one of the twenty standard amino acids and the only one whose side chain loops back to bond with its own backbone nitrogen, forming a rigid five-membered ring. That structure makes proline a disruptor of alpha helices and beta sheets.
• Purification methods (chemistry)Purification methods in chemistry are laboratory techniques — extraction, distillation, chromatography, recrystallization, and filtration — that separate a desired compound from a mixture based on differences in physical properties.
• Recombinant DNARecombinant DNA is a DNA molecule assembled in the laboratory from sequences that do not occur together in nature, typically by cutting DNA with restriction enzymes and joining the pieces with DNA ligase. Inserting it into a host cell allows the sequence to be copied or expressed.
• RetrovirusA retrovirus is an RNA virus that uses the enzyme reverse transcriptase to copy its genome into DNA, which is then integrated into the host cell's chromosome. HIV and HTLV are the best-known human retroviruses.
• Rotational kinematicsRotational kinematics describes the motion of rotating objects using angular position, angular velocity, and angular acceleration. Its equations mirror the linear kinematic equations, with each linear quantity replaced by its angular counterpart.
• Signal transductionSignal transduction is the process by which a cell converts an external chemical or physical signal into an internal response. A ligand binds a receptor, the receptor activates intracellular messengers, and a cascade amplifies the message into a change in cell behavior.
• Simple harmonic motionSimple harmonic motion is the back-and-forth motion that occurs when the restoring force on an object is proportional to its displacement from equilibrium and directed back toward it. A mass on a spring and a small-angle pendulum are the standard examples.
• Soil classificationSoil classification is the process of grouping soils into standard categories based on grain size distribution and plasticity so engineers can predict how they will behave. The Unified Soil Classification System and the AASHTO system are the two most widely used schemes.
• StaticsStatics is the branch of mechanics that studies bodies at rest or moving at constant velocity, where all forces and moments balance. Its governing conditions are that the sum of forces equals zero and the sum of moments equals zero.
• StereochemistryStereochemistry is the study of how atoms are arranged in three-dimensional space within molecules and how that arrangement affects chemical and physical properties. Two molecules with identical connectivity but different spatial arrangements are stereoisomers, and they can behave very differently.
• StoichiometryStoichiometry is the use of a balanced chemical equation to calculate the quantities of reactants consumed and products formed. Its central tool is the mole ratio taken directly from the equation's coefficients.
• TerpenesTerpenes are a class of lipids built from repeating five-carbon isoprene units. They range from the small volatile molecules that give plants their scents to large structures such as squalene, the precursor of cholesterol and the steroid hormones.
• The periodic tableThe periodic table is an arrangement of the chemical elements in order of increasing atomic number, organized into rows and columns so that elements with similar properties line up. Its structure reflects the electron configurations of the atoms.
• ThermochemistryThermochemistry is the study of the heat absorbed or released during chemical reactions and physical changes. It uses quantities such as enthalpy, entropy, and Gibbs free energy to predict whether a process gives off energy and whether it happens spontaneously.
• TitrationTitration is a laboratory technique for finding an unknown concentration by gradually adding a solution of known concentration (the titrant) until the reaction is complete. The endpoint is signaled by an indicator or a pH meter.
• Torque and rotational inertiaTorque is the rotational effect of a force, equal to τ = rF sin θ — the force times its lever arm about a pivot. Rotational inertia (moment of inertia) measures an object's resistance to changes in its rotation.
• Translation (biology)Translation is the process by which ribosomes read messenger RNA (mRNA) and assemble the corresponding sequence of amino acids into a protein. It is the second step of gene expression, following transcription.
• Translational equilibriumTranslational equilibrium is the state in which the net force on an object is zero, so it has no linear acceleration. The object is either at rest or moving in a straight line at constant velocity.
• Venturi effectThe Venturi effect is the drop in fluid pressure that occurs when a fluid speeds up while flowing through a constricted section of pipe. It follows from the continuity equation and Bernoulli's principle.
• Visible light spectrumThe visible light spectrum is the narrow band of electromagnetic radiation the human eye can detect, spanning roughly 400 to 700 nanometers. Violet sits at the short-wavelength, high-energy end and red at the long-wavelength, low-energy end.
• Water treatment and distributionWater treatment is the sequence of processes — coagulation, flocculation, sedimentation, filtration, and disinfection — that makes raw water safe to drink, and distribution is the network of pumps, storage, and pipes that delivers it to users.
• Work and powerIn physics, work is the energy transferred when a force moves an object through a distance (W = Fd cos θ), and power is the rate at which that work is done (P = W/t). Work is measured in joules and power in watts.
• Work, energy, and powerWork is energy transferred when a force moves an object, energy is the capacity to do work, and power is the rate at which work is done. Together they form the framework physics uses to track how energy moves and transforms.