ACT Science

ACT Science Terms: Complete Vocabulary Reference

The ACT Science section tests reading comprehension of scientific passages — but students who know the vocabulary read faster and answer more accurately. This reference covers 250+ terms across research methodology, graphs, biology, chemistry, physics, and earth science, each with a definition and example of how it appears in ACT passages.

250+ terms · 6 subject areas · ACT passage examples

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Why vocabulary matters on ACT Science:

The ACT Science section does not require scientific knowledge — it tests data analysis and reading comprehension. However, students who do not know basic scientific vocabulary waste time re-reading passages to figure out terms like “control group,” “dependent variable,” “correlation,” or “pH.” Knowing these terms allows you to read passages faster and focus on the data and questions.

Know the difference between hypothesis, theory, and conclusion

Understand what a control group is and why it matters

Read graphs confidently without stopping to puzzle over axis labels

Data & Research Methodology Terms

These terms appear in almost every ACT Science passage. They describe how experiments are designed, how data are interpreted, and the difference between a good and a flawed conclusion.

#	Term	Definition	How It Appears in an ACT Passage
1	hypothesis	A proposed explanation for an observation, made before testing. Must be testable and falsifiable.	The researchers hypothesized that increased temperature would increase enzyme activity up to 40°C.
2	theory	A well-substantiated explanation supported by extensive evidence from many independent sources. Not a guess.	The passage presents findings consistent with the theory of plate tectonics.
3	independent variable	The variable the experimenter deliberately changes or controls. Plotted on the x-axis.	In Experiment 1, temperature was the independent variable, ranging from 0°C to 100°C.
4	dependent variable	The variable measured to see how it responds to changes in the independent variable. Plotted on the y-axis.	Enzyme reaction rate was the dependent variable, measured in micromoles per minute.
5	controlled variable	A variable that is kept constant so it does not affect the dependent variable.	pH was held constant at 7.0 across all trials to eliminate it as a confounding variable.
6	control group	The group in an experiment that does not receive the treatment; provides a baseline for comparison.	Group A received no treatment and served as the control group.
7	experimental group	The group that receives the treatment or manipulation being tested.	Groups B, C, and D received increasing doses of the compound and were compared to the control.
8	confounding variable	An uncontrolled variable that affects the dependent variable and could distort the results.	The authors noted that diet was a potential confounding variable not accounted for in the study design.
9	replication	Repeating an experiment to verify results; also copying conditions across multiple trials.	Each trial was replicated three times to reduce the effect of random error.
10	reproducibility	The ability of independent researchers to obtain the same results using the same methods.	The study's low reproducibility rate raised questions about the reliability of the findings.
11	sample size	The number of subjects or observations in a study. Larger samples yield more reliable results.	With a sample size of only 12 mice, the results may not generalize to the broader population.
12	bias	A systematic error in experimental design, sampling, or data interpretation that skews results.	Self-reported data introduced a potential recall bias into the study.
13	statistical significance	A result is statistically significant if it is unlikely to have occurred by chance (typically p < 0.05).	The difference between Groups A and B was statistically significant (p = 0.02).
14	p-value	The probability that the observed results occurred by chance. Lower p-values indicate stronger evidence.	A p-value below 0.05 indicates that the result is unlikely to be due to random variation.
15	correlation	A statistical relationship between two variables; they change together. Does not imply causation.	Figure 2 shows a positive correlation between atmospheric CO₂ and average global temperature.
16	causation	One variable directly causes a change in another. Requires controlled experimentation to establish.	The passage cautions that correlation between two variables does not establish causation.
17	error bar	A graphical representation of variability in data; indicates the range of uncertainty around a data point.	The overlapping error bars in Figure 1 suggest the difference between groups may not be significant.
18	standard deviation	A measure of how spread out data points are from the mean. Larger SD = more variability.	The large standard deviation in Trial 3 suggests inconsistency in the experimental conditions.
19	mean (average)	The sum of all values divided by the number of values.	The mean reaction time across all trials was 4.3 seconds.
20	median	The middle value in an ordered dataset; less affected by extreme outliers than the mean.	Because the dataset contained several extreme values, the researchers reported the median rather than the mean.
21	outlier	A data point that differs significantly from other observations in a dataset.	The single outlier in Figure 3 was removed after investigators determined it resulted from equipment error.
22	validity	The extent to which a study measures what it claims to measure.	Critics questioned the validity of using self-reported pain scores as an objective measure.
23	reliability	The consistency of a measurement — whether the same method produces the same results repeatedly.	The high inter-rater reliability confirmed that different observers categorized behaviors consistently.
24	conclusion	A statement that explains what the data show; must be directly supported by the evidence.	Based on the data in Table 2, the authors conclude that compound X inhibits bacterial growth at concentrations above 5 mg/L.
25	inference	A conclusion drawn from data that goes beyond what is directly observed.	From the data, it can be inferred that decreasing light intensity reduces the rate of photosynthesis.
26	interpolation	Estimating a value within the range of known data points.	Using Figure 1, interpolate the expected reaction rate at 35°C.
27	extrapolation	Estimating a value beyond the range of known data — less reliable than interpolation.	Extrapolating from the graph suggests that reaction rate continues to decline below 10°C, though this was not directly tested.
28	qualitative data	Descriptive, non-numerical data based on observations (color, texture, behavior).	Researchers noted qualitative changes in cell morphology, including irregular shape and fragmented membranes.
29	quantitative data	Numerical data that can be measured and expressed in numbers.	Quantitative data from the spectrophotometer showed absorbance values between 0.2 and 1.4 AU.

Graph & Table Vocabulary

Most ACT Science questions reference a figure or table. Knowing this vocabulary precisely eliminates hesitation when reading a graph.

#	Term	Definition	How It Appears in an ACT Passage
1	x-axis (horizontal axis)	The horizontal axis of a graph; typically represents the independent variable.	In Figure 2, the x-axis represents time in seconds.
2	y-axis (vertical axis)	The vertical axis of a graph; typically represents the dependent variable.	The y-axis shows enzyme activity in micromoles per minute.
3	data point	A single plotted value on a graph representing one observation.	Each data point in Figure 1 represents the mean of three trials.
4	trend	The general direction in which data moves (increasing, decreasing, constant).	Figure 3 shows a clear upward trend as temperature increases from 20°C to 40°C.
5	direct (positive) relationship	As one variable increases, the other also increases.	The graph shows a direct relationship: as pH increased, bacterial growth rate increased.
6	inverse (negative) relationship	As one variable increases, the other decreases.	There is an inverse relationship between depth and light intensity in the ocean.
7	plateau	A region of a graph where the dependent variable stops changing despite changes in the independent variable.	Above 60°C, the reaction rate reached a plateau, suggesting the enzyme had been denatured.
8	range	The spread between the minimum and maximum values in a dataset.	The range of test scores was 42 to 98, giving a range of 56 points.
9	scale	The intervals used on the axes of a graph; determines how data appear visually.	Students should note that the y-axis in Figure 2 does not start at zero, which may make differences appear larger than they are.
10	legend / key	An explanatory table or list that identifies the symbols, colors, or patterns used in a figure.	According to the legend, the solid line represents Group A and the dashed line represents Group B.
11	bar graph	A graph using rectangular bars of varying height to compare quantities across categories.	The bar graph in Figure 1 compares the mean weights of five different species.
12	line graph	A graph connecting data points with a line; best for showing change over time or continuous variables.	The line graph shows temperature change over a 24-hour period.
13	scatter plot	A graph where individual data points are plotted without connecting lines; used to show relationships between two variables.	The scatter plot reveals no clear relationship between age and recovery time in this sample.
14	anomaly	A data point or result that does not fit the expected pattern.	The anomaly at 50°C — where reaction rate dropped unexpectedly — was not explained by the researchers.
15	table	An organized display of data in rows and columns; precise values, not visual trends.	Table 1 provides the exact absorbance values measured at each wavelength.

Biology Terms (50+)

Biology is the most frequently represented subject in ACT Science passages. Cell biology, genetics, ecology, and evolution appear regularly.

#	Term	Definition	How It Appears in an ACT Passage
1	cell	The fundamental unit of life; the smallest structure capable of performing all life functions.	Experiment 1 measured the rate at which red blood cells ruptured in hypotonic solutions.
2	cell membrane	A selectively permeable phospholipid bilayer that surrounds and encloses a cell.	The cell membrane's permeability to certain ions was altered by the compound under investigation.
3	nucleus	The membrane-bound organelle containing DNA; the control center of the cell.	After treatment, the nucleus showed irregular staining patterns suggesting DNA fragmentation.
4	mitochondria	Organelles that produce ATP through cellular respiration; the cell's energy factories.	Cells with more mitochondria showed greater aerobic capacity in the experimental trials.
5	DNA (deoxyribonucleic acid)	The molecule carrying genetic information; composed of nucleotide base pairs in a double helix.	The researchers used gel electrophoresis to separate DNA fragments by size.
6	RNA (ribonucleic acid)	A molecule involved in decoding DNA instructions and synthesizing proteins (mRNA, tRNA, rRNA).	mRNA transcripts were quantified to assess gene expression levels in each tissue sample.
7	protein	A large molecule made of amino acids; performs structural, enzymatic, transport, and regulatory functions.	The protein's tertiary structure was disrupted when the temperature exceeded 60°C.
8	enzyme	A biological catalyst (usually a protein) that speeds up chemical reactions without being consumed.	Amylase is the enzyme that catalyzes the breakdown of starch into simple sugars.
9	photosynthesis	The process by which plants, algae, and some bacteria convert light energy into chemical energy (glucose) using CO₂ and water.	Plants exposed to blue light showed the highest photosynthesis rates in Study 1.
10	cellular respiration	The metabolic process by which cells break down glucose to produce ATP; can be aerobic or anaerobic.	In the absence of oxygen, the yeast switched from aerobic to anaerobic respiration.
11	aerobic respiration	Cellular respiration that requires oxygen and produces CO₂, water, and ATP; more efficient than anaerobic.	The aerobic organisms produced significantly more ATP per glucose molecule than the anaerobic strains.
12	anaerobic respiration	Cellular respiration that occurs without oxygen; produces lactic acid or ethanol; less efficient.	When deprived of oxygen, muscle cells shifted to anaerobic respiration and accumulated lactic acid.
13	osmosis	Diffusion of water molecules across a semipermeable membrane from regions of low to high solute concentration.	In Experiment 2, cells placed in a hypertonic solution lost water through osmosis and shrank.
14	diffusion	Movement of particles from regions of high to low concentration; requires no energy.	Oxygen diffuses across the alveolar membrane into the bloodstream down its concentration gradient.
15	active transport	Movement of molecules across a membrane against their concentration gradient; requires ATP.	The sodium-potassium pump uses active transport to maintain ion gradients across the cell membrane.
16	mitosis	Cell division producing two genetically identical daughter cells with the same chromosome number.	Cancerous cells entered mitosis at a rate five times higher than normal tissue.
17	meiosis	Cell division producing four genetically diverse gametes with half the chromosome number.	During meiosis, crossing-over occurs, increasing the genetic diversity of gametes.
18	chromosome	A thread-like structure in the cell nucleus carrying genetic information (DNA + proteins).	Humans have 46 chromosomes arranged in 23 pairs.
19	gene	A segment of DNA that codes for a specific protein or functional RNA.	Researchers isolated the gene responsible for producing the light-sensing pigment.
20	allele	One of two or more versions of a gene at a given locus; may be dominant or recessive.	Each parent carried one dominant and one recessive allele for eye color.
21	genotype	An organism's full set of genes or the specific genetic makeup at a locus.	All offspring with the genotype Bb expressed the dominant phenotype.
22	phenotype	The observable physical and behavioral characteristics of an organism, resulting from genotype and environment.	Identical genotypes produced different phenotypes when grown under different light conditions.
23	natural selection	The process by which heritable traits that increase reproductive success become more common over generations.	The passage describes how antibiotic resistance evolves by natural selection.
24	evolution	Change in allele frequencies in a population over time; descent with modification.	The fossil record provides evidence of the evolution of the horse over 55 million years.
25	species	A group of organisms that can interbreed and produce fertile offspring.	The two populations were classified as separate species after researchers confirmed reproductive isolation.
26	ecosystem	A biological community of interacting organisms and their physical environment.	The introduction of wolves into Yellowstone transformed the entire ecosystem by altering elk grazing patterns.
27	food chain / food web	A linear or network representation of energy flow from producers to successive consumers.	Figure 2 shows the food web in the estuary, with phytoplankton at the base.
28	producer (autotroph)	An organism that produces its own food from sunlight or chemicals; the base of most food chains.	Plants and algae are producers, converting solar energy into biomass.
29	consumer (heterotroph)	An organism that obtains energy by consuming other organisms.	Primary consumers eat producers directly; secondary consumers eat primary consumers.
30	predator / prey	Predator: an organism that hunts and eats other organisms. Prey: an organism that is hunted.	When predator populations declined, prey populations increased sharply.
31	habitat	The natural environment in which an organism lives.	Loss of forest habitat has fragmented populations of the spotted owl.
32	biodiversity	The variety of life forms in a given area; includes species, genetic, and ecosystem diversity.	Figure 1 shows that biodiversity declined as land was converted to agriculture.
33	adaptation	A heritable trait that increases an organism's fitness in its environment.	The thick fur of Arctic mammals is an adaptation to cold environments.
34	mutation	A change in the DNA sequence; can be beneficial, harmful, or neutral.	The resistance to the antibiotic arose from a point mutation in the bacterial genome.
35	heredity	The passing of genetic traits from parents to offspring.	The passage describes how Mendel's experiments revealed the laws of heredity.
36	homeostasis	The maintenance of stable internal conditions in a living organism despite external changes.	When ambient temperature dropped, the mammal's metabolic rate increased to maintain homeostasis.
37	stimulus / response	Stimulus: a change in the environment. Response: the organism's reaction to the stimulus.	The plant showed a phototropic response, bending toward the light stimulus.
38	taxonomy	The science of classifying organisms into a hierarchical system (domain, kingdom, phylum, class, order, family, genus, species).	Based on new genetic data, the organism was reclassified within a different taxonomic order.
39	symbiosis	A close and long-term biological interaction between two different organisms.	The passage describes the symbiotic relationship between the nitrogen-fixing bacteria and legume roots.
40	parasite / host	Parasite: an organism that benefits at the expense of another. Host: the organism that is harmed.	The parasite invaded host cells and hijacked their replication machinery.
41	population	All the individuals of the same species living in a defined area at the same time.	The population of breeding pairs declined by 40% over the study period.
42	carrying capacity	The maximum population size an environment can support given available resources.	The population leveled off near the carrying capacity estimated at 5,000 individuals.
43	succession	The gradual process by which an ecosystem changes and develops over time.	After the fire, the researchers tracked ecological succession from bare ground to mature forest.
44	fermentation	An anaerobic metabolic process that converts sugars to acids, gases, or alcohol.	Yeast fermentation produced carbon dioxide and ethanol when the flasks were sealed.
45	transpiration	The process by which water moves through a plant and evaporates from leaves.	Transpiration rates increased significantly when temperature rose from 20°C to 35°C.
46	hormone	A chemical messenger produced in one part of an organism that regulates functions elsewhere.	Insulin, a hormone produced in the pancreas, regulates blood glucose levels.
47	neuron	A nerve cell that transmits electrical impulses throughout the nervous system.	The compound blocked sodium channels, preventing neurons from firing.
48	ATP (adenosine triphosphate)	The primary energy currency of cells; released when a phosphate group is removed.	Each glucose molecule yields approximately 36–38 ATP molecules through aerobic respiration.
49	plasmid	A small, circular DNA molecule in bacteria separate from the main chromosome; often carries antibiotic resistance genes.	Researchers introduced the target gene into bacterial cells using a plasmid vector.
50	gel electrophoresis	A laboratory technique that separates DNA, RNA, or proteins by size using an electric field through a gel.	Gel electrophoresis of the PCR products revealed bands at the expected fragment sizes.

Chemistry Terms (50+)

Chemistry passages on the ACT test your ability to interpret reaction data, concentration effects, and pH changes. These terms are essential.

#	Term	Definition	How It Appears in an ACT Passage
1	element	A pure substance made of only one kind of atom; cannot be broken down by chemical means.	Carbon is an element with atomic number 6 that forms the backbone of organic molecules.
2	compound	A substance made of two or more different elements chemically bonded together in fixed ratios.	Water (H₂O) is a compound formed when hydrogen and oxygen bond covalently.
3	mixture	Two or more substances physically combined but not chemically bonded; components retain their properties.	Air is a mixture of nitrogen, oxygen, argon, and trace gases.
4	atom	The smallest unit of an element that retains the chemical properties of that element.	Each atom of carbon has 6 protons in its nucleus.
5	molecule	Two or more atoms bonded together; the smallest unit of a compound that retains its chemical properties.	A water molecule consists of two hydrogen atoms bonded to one oxygen atom.
6	ion	An atom or molecule that has lost or gained electrons, giving it a positive or negative charge.	Sodium ions (Na⁺) move through protein channels across the cell membrane.
7	atomic number	The number of protons in the nucleus of an atom; defines the element.	Elements are arranged by increasing atomic number in the periodic table.
8	isotope	Atoms of the same element with different numbers of neutrons; same atomic number, different mass number.	Carbon-12 and Carbon-14 are isotopes; Carbon-14 is radioactive and used in dating.
9	pH	A logarithmic scale measuring the acidity or basicity of a solution; 0–14, with 7 as neutral.	The enzyme functioned optimally at pH 7.4, the pH of human blood.
10	acid	A substance that donates H⁺ ions; pH below 7.	Adding hydrochloric acid to the solution lowered its pH from 7 to 3.
11	base (alkali)	A substance that accepts H⁺ ions or donates OH⁻; pH above 7.	Sodium hydroxide is a strong base that rapidly increased the solution's pH.
12	neutralization	A reaction between an acid and a base that produces water and a salt.	When equal moles of HCl and NaOH were mixed, neutralization produced NaCl and water.
13	concentration	The amount of solute dissolved in a given volume of solution; measured in mol/L (molarity).	As substrate concentration increased, reaction rate increased up to a plateau.
14	solute	The substance dissolved in a solution (e.g., salt in saltwater).	NaCl was the solute in the saline solution used in Experiment 3.
15	solvent	The substance in which the solute is dissolved (e.g., water in saltwater).	Water is the universal biological solvent due to its polarity.
16	solution	A homogeneous mixture of solute and solvent.	The researchers prepared a buffered solution to maintain constant pH throughout the trials.
17	solubility	The maximum amount of solute that can dissolve in a given quantity of solvent at a specific temperature.	Solubility of oxygen in water decreases as water temperature rises.
18	catalyst	A substance that increases the rate of a reaction without being consumed.	Platinum serves as an industrial catalyst in the synthesis of ammonia.
19	activation energy	The minimum energy required for a chemical reaction to occur.	Enzymes lower the activation energy, allowing reactions to proceed faster at body temperature.
20	exothermic reaction	A reaction that releases heat energy to the surroundings; products have less energy than reactants.	Combustion is exothermic; the temperature of the surrounding water increased during the reaction.
21	endothermic reaction	A reaction that absorbs heat energy from the surroundings; products have more energy than reactants.	The dissolving of ammonium nitrate in water is endothermic, causing the temperature to drop.
22	oxidation	Loss of electrons (or gain of oxygen); opposite of reduction.	Iron undergoes oxidation in the presence of oxygen and water, producing rust (Fe₂O₃).
23	reduction	Gain of electrons (or loss of oxygen); opposite of oxidation.	In the reaction, copper ions were reduced to copper metal at the cathode.
24	redox reaction	A reaction involving both oxidation and reduction; electrons transfer from one reactant to another.	The electrochemical cell performed a redox reaction, generating an electric current.
25	equilibrium	A state in which forward and reverse reaction rates are equal; concentrations remain constant.	Adding more reactant shifted the equilibrium to the right, producing more product.
26	Le Chatelier's principle	When a system at equilibrium is disturbed, it shifts to partially counteract the disturbance.	According to Le Chatelier's principle, increasing pressure shifted the equilibrium toward the side with fewer gas molecules.
27	molar mass	The mass of one mole of a substance; equal to the atomic/molecular weight in grams.	With a molar mass of 18 g/mol, water is one of the lightest molecules.
28	mole	A unit equal to 6.02 × 10²³ particles; the standard unit for measuring amounts of substance.	The reaction required 2 moles of hydrogen gas for every mole of oxygen.
29	stoichiometry	The calculation of quantities of reactants and products in chemical reactions using balanced equations.	Stoichiometric calculations showed that 4 grams of oxygen reacted with 1 gram of hydrogen.
30	molarity (M)	The number of moles of solute per liter of solution.	A 2 M NaCl solution contains 2 moles of NaCl per liter of water.
31	precipitation	The formation of an insoluble solid when two solutions are mixed.	Mixing the two clear solutions caused immediate precipitation of a white solid.
32	distillation	A process to separate mixtures by differences in boiling point.	Distillation separated the two liquid components because they differed by 40°C in boiling point.
33	chromatography	A technique for separating mixtures based on differences in how components adhere to a stationary phase.	Paper chromatography separated the pigments in the plant extract into four distinct bands.
34	spectrophotometry	Measuring how much light a substance absorbs at different wavelengths; used to determine concentration.	Absorbance at 600 nm was measured using a spectrophotometer to determine cell density.
35	empirical formula	The simplest whole-number ratio of atoms in a compound.	The empirical formula CH₂O indicates a 1:2:1 ratio of carbon, hydrogen, and oxygen.
36	molecular formula	The actual number of atoms of each element in a molecule.	Glucose has the molecular formula C₆H₁₂O₆.
37	polarity	An unequal distribution of charge in a molecule, resulting in partial positive and negative ends.	Water's polarity allows it to dissolve ionic compounds and polar molecules.
38	hydrogen bond	A weak electrostatic attraction between a hydrogen atom bonded to a highly electronegative atom and another electronegative atom.	Hydrogen bonds between water molecules give water its unusually high boiling point.
39	covalent bond	A chemical bond formed when atoms share electron pairs.	Carbon forms four covalent bonds, enabling the formation of complex organic molecules.
40	ionic bond	A chemical bond formed by the electrostatic attraction between oppositely charged ions.	Sodium and chloride form ionic bonds in sodium chloride.
41	titration	A quantitative analytical method in which a solution of known concentration is used to determine the concentration of an unknown.	The researcher used acid-base titration to determine the concentration of the acetic acid sample.
42	half-life	The time required for half of a radioactive isotope's atoms to decay.	With a half-life of 5,730 years, Carbon-14 is useful for dating organic materials up to 50,000 years old.
43	buffer	A solution that resists changes in pH when small amounts of acid or base are added.	The phosphate buffer maintained a constant pH of 7.4 throughout all three experiments.
44	entropy	A measure of disorder or randomness in a system; tends to increase in spontaneous processes.	The dissolution of the crystal increased the system's entropy as ions dispersed throughout the solution.
45	Gibbs free energy	A thermodynamic quantity predicting whether a reaction is spontaneous; negative ΔG = spontaneous.	The negative ΔG value confirmed that the reaction would proceed spontaneously under standard conditions.
46	polymer	A large molecule made of many repeated smaller units (monomers) bonded together.	Cellulose is a structural polymer of glucose units found in plant cell walls.
47	monomer	A small molecule that can join with others to form a polymer.	Amino acids are the monomers of proteins; nucleotides are the monomers of DNA and RNA.
48	hydrolysis	A reaction in which a molecule is broken apart by the addition of water.	Proteins are broken down into amino acids by hydrolysis during digestion.
49	condensation (dehydration) reaction	A reaction in which two molecules join with the loss of water.	The formation of peptide bonds occurs through condensation reactions, releasing water.
50	electronegativity	The tendency of an atom to attract electrons in a chemical bond; increases toward the upper-right of the periodic table.	Oxygen's high electronegativity makes it the negative end of the water dipole.
51	rate of reaction	The speed at which reactants are converted to products; influenced by temperature, concentration, and catalysts.	Table 1 shows that doubling the substrate concentration approximately doubled the rate of reaction.

Physics Terms (50+)

Physics passages commonly involve motion, forces, waves, and electricity. ACT Physics questions often ask about relationships between variables shown in graphs.

#	Term	Definition	How It Appears in an ACT Passage
1	velocity	The rate of change of position in a specific direction; a vector quantity (speed + direction).	The object's initial velocity was 10 m/s due north.
2	speed	The rate of change of position; a scalar quantity (magnitude only, no direction).	The car's average speed over the journey was 60 km/h.
3	acceleration	The rate of change of velocity; can involve change in speed or direction.	Figure 1 shows constant acceleration as velocity increased linearly from 0 to 20 m/s over 4 seconds.
4	force	An interaction that changes or tends to change the motion of an object; measured in Newtons.	The net force on the object was calculated by subtracting friction from the applied force.
5	mass	The amount of matter in an object; measured in kilograms; different from weight.	Objects with greater mass require greater force to achieve the same acceleration (F = ma).
6	weight	The force exerted on an object by gravity; weight = mass × gravitational acceleration.	The object's weight on Earth is 9.8 N for each kilogram of mass.
7	inertia	The resistance of an object to changes in its state of motion; proportional to mass.	Newton's first law states that objects remain at rest or in uniform motion due to inertia.
8	momentum	The product of mass and velocity (p = mv); conserved in closed systems.	After the collision, the total momentum of the system was conserved.
9	friction	A force that opposes the relative motion between two surfaces in contact.	Adding lubricant between the surfaces significantly reduced friction and heat generation.
10	gravity	The force of attraction between any two objects with mass; decreases with the square of distance.	The gravitational force between the objects decreased as the distance between them doubled.
11	work	Energy transferred by a force over a distance; W = F × d × cos(θ).	The crane performed 500 joules of work lifting the load 5 meters.
12	energy	The capacity to do work; conserved in a closed system (conservation of energy).	As the pendulum swings, energy converts between kinetic and potential forms.
13	kinetic energy	Energy of motion; KE = ½mv².	When speed doubled, kinetic energy quadrupled, as predicted by the equation KE = ½mv².
14	potential energy	Stored energy due to position or configuration; gravitational PE = mgh.	At maximum height, all kinetic energy had been converted to gravitational potential energy.
15	power	The rate of doing work or transferring energy; P = W/t; measured in watts.	The motor produced 200 watts of power, completing the task in half the time.
16	wave	A disturbance that transfers energy from one place to another without transferring matter.	The wave traveled through the medium at a constant speed of 340 m/s.
17	frequency	The number of complete wave cycles per unit time; measured in hertz (Hz).	Figure 2 shows that pitch increases as the frequency of the sound wave increases.
18	wavelength	The distance between two consecutive identical points on a wave (e.g., crest to crest).	Red light has a longer wavelength than blue light.
19	amplitude	The maximum displacement of a wave from its equilibrium position; related to energy.	Increasing the amplitude of the wave increased the energy it carried but did not change its speed.
20	electromagnetic spectrum	The range of all electromagnetic radiation, from radio waves (low frequency) to gamma rays (high frequency).	Ultraviolet radiation, part of the electromagnetic spectrum, has enough energy to damage DNA.
21	reflection	The bouncing of waves off a surface; the angle of incidence equals the angle of reflection.	The laser beam underwent total internal reflection at the interface between the two media.
22	refraction	The bending of waves as they pass from one medium into another with different density.	The pencil appeared bent in the glass of water due to refraction at the water-air interface.
23	electrical resistance	The opposition to the flow of electrical current; measured in ohms (Ω).	The resistance of the filament increased as its temperature rose.
24	current (electric)	The flow of electric charge; measured in amperes (A).	Doubling the voltage caused the current through the resistor to double, consistent with Ohm's law.
25	voltage (potential difference)	The difference in electric potential energy per unit charge between two points; measured in volts (V).	The battery maintained a constant voltage of 9V across the circuit.
26	Ohm's law	V = IR; voltage equals current multiplied by resistance.	Using Ohm's law, the team calculated the expected current before connecting the circuit.
27	circuit	A closed path through which electric current can flow.	When the switch was opened, the circuit was broken and current stopped flowing.
28	pressure	Force applied per unit area; P = F/A; measured in pascals (Pa).	As depth increased, water pressure increased linearly, as shown in Figure 1.
29	density	Mass per unit volume; ρ = m/V; measured in kg/m³ or g/cm³.	The substance sank because its density exceeded that of the surrounding liquid.
30	buoyancy	The upward force exerted by a fluid on an object immersed in it; equal to the weight of displaced fluid.	The object floated because the buoyant force exceeded its weight.
31	half-life (physics)	The time for half of a radioactive sample to decay; constant for a given isotope.	After two half-lives, one-quarter of the original radioactive material remained.
32	nuclear fission	The splitting of a heavy nucleus into lighter nuclei, releasing energy.	Nuclear fission of uranium-235 releases approximately 200 MeV per reaction.
33	nuclear fusion	The combining of light nuclei into a heavier nucleus, releasing large amounts of energy.	The sun produces energy through nuclear fusion of hydrogen into helium in its core.
34	electromagnetic induction	The generation of an electric current by a changing magnetic field.	Faraday demonstrated electromagnetic induction by moving a magnet through a coil of wire.
35	thermodynamics	The branch of physics dealing with heat, work, and temperature and their relation to energy.	The first law of thermodynamics states that energy can neither be created nor destroyed.
36	heat transfer	Movement of thermal energy; can occur by conduction, convection, or radiation.	Heat transferred from the hot plate to the liquid through conduction.
37	specific heat capacity	The amount of energy required to raise 1 kg of a substance by 1°C.	Water's high specific heat capacity (4,186 J/kg°C) explains why it resists temperature changes.
38	optics	The study of light and its interaction with matter, including reflection, refraction, and diffraction.	The optics experiment measured how different materials bent light at varying angles.
39	diffraction	The bending of waves around obstacles or through openings; more pronounced when wavelength ≈ opening size.	The diffraction pattern in Figure 3 confirmed the wave nature of the particle beam.
40	photon	A discrete particle (quantum) of light energy; energy proportional to frequency.	Each photon absorbed by the photoelectric material released one electron from the surface.
41	nuclear radiation	Particles or energy emitted from an unstable nucleus; types include alpha, beta, and gamma.	Alpha radiation is stopped by a sheet of paper; gamma radiation requires lead shielding.
42	projectile motion	The motion of an object launched into the air under the influence of gravity alone.	The projectile followed a parabolic path, consistent with the equations for projectile motion.
43	torque	The rotational equivalent of force; the tendency of a force to rotate an object about an axis.	Increasing the length of the lever arm doubled the torque applied to the bolt.
44	centripetal force	The net force directed toward the center of a circular path, keeping an object in circular motion.	The string provided the centripetal force keeping the ball in circular motion.
45	resonance	The tendency of a system to oscillate at greater amplitude at certain frequencies.	When the driving frequency matched the natural frequency, resonance caused the amplitude to spike.
46	interference	The combination of two waves to form a resultant wave; can be constructive or destructive.	Constructive interference produced regions of maximum amplitude, while destructive interference created nodal lines.
47	conservation laws	Physical laws stating that certain quantities (mass, energy, momentum, charge) remain constant in closed systems.	The collision satisfied the conservation of momentum: total momentum before equaled total momentum after.
48	signal-to-noise ratio	The ratio of the desired signal to background noise; higher ratios indicate cleaner data.	The signal-to-noise ratio improved when the researchers shielded the apparatus from electromagnetic interference.
49	equilibrium (physics)	A state in which an object has no net force acting on it; either at rest or moving at constant velocity.	The beam was in static equilibrium when torques on both sides of the fulcrum were equal.
50	displacement	The change in position of an object; a vector quantity specifying magnitude and direction.	Although the runner traveled 400 m, her displacement was 0 m because she returned to the start.

Earth Science & Astronomy Terms (40+)

Earth science and astronomy passages appear in most ACT Science tests. These passages often involve geological time, rock types, atmospheric processes, and space phenomena.

#	Term	Definition	How It Appears in an ACT Passage
1	tectonic plate	A large segment of Earth's lithosphere that moves due to convection in the mantle.	The earthquake occurred along the boundary where two tectonic plates collide.
2	earthquake	A sudden release of energy in Earth's crust causing ground shaking; measured on the Richter scale.	The seismograph recorded an earthquake of magnitude 6.2, centered 10 km below the surface.
3	erosion	The wearing away of soil, rock, or other material by wind, water, or ice.	Coastal erosion has accelerated as sea levels rise and storm frequency increases.
4	sediment	Solid material (rock, sand, organic material) deposited by wind, water, or glaciers.	Analysis of sediment cores revealed layers deposited during each ice age.
5	sedimentary rock	Rock formed from the compaction and cementation of sediment layers.	The sedimentary rock layers provided a 300-million-year record of environmental change.
6	igneous rock	Rock formed from the cooling and solidification of magma or lava.	The igneous rock contained crystals indicating it cooled slowly underground.
7	metamorphic rock	Rock formed from existing rock transformed by heat and pressure.	The marble, a metamorphic rock, formed when limestone was exposed to intense heat and pressure.
8	fossil	The preserved remains or trace of an ancient organism.	The fossil record shows that the species went extinct approximately 65 million years ago.
9	stratigraphy	The study of rock layers (strata) and their chronological sequence.	Using stratigraphy, the geologists determined that the upper layer was younger than the lower layer.
10	geologic time scale	A chronological framework used by geologists to describe Earth's history, divided into eons, eras, periods, and epochs.	The passage references the Cretaceous period of the Mesozoic era on the geologic time scale.
11	radiometric dating	Calculating the age of materials by measuring the decay of radioactive isotopes.	Uranium-lead radiometric dating placed the age of the zircon crystals at 4.1 billion years.
12	atmosphere	The layers of gas surrounding Earth; composed primarily of nitrogen (78%) and oxygen (21%).	The atmosphere blocks most ultraviolet radiation before it reaches Earth's surface.
13	greenhouse effect	The trapping of heat in the atmosphere by gases such as CO₂, methane, and water vapor.	Increased atmospheric CO₂ enhances the greenhouse effect, raising global average temperatures.
14	climate	The average weather conditions in a region over long periods (typically 30 years or more).	The passage distinguishes between climate (long-term patterns) and weather (short-term conditions).
15	ocean current	A continuous, directed movement of seawater generated by wind, water density differences, and Earth's rotation.	The disruption of ocean currents in the North Atlantic could alter climate patterns in Europe.
16	water cycle (hydrological cycle)	The continuous movement of water through evaporation, condensation, precipitation, and runoff.	Figure 1 illustrates the water cycle, showing the pathways of water through the atmosphere and surface.
17	watershed	An area of land that drains into a particular river, lake, or other body of water.	Deforestation within the watershed increased runoff and reduced water quality downstream.
18	biome	A large community of plants and animals occupying a major habitat defined by climate.	The passage compares biodiversity across six major terrestrial biomes from tropical rainforest to tundra.
19	solar radiation	Energy emitted by the sun; drives Earth's climate, weather, and photosynthesis.	Variations in solar radiation output over decades can influence global temperature.
20	orbit	The curved path of a celestial object around another due to gravity.	Earth's elliptical orbit means it is slightly closer to the sun in January than in July.
21	planet	A large celestial body that orbits a star, has cleared its orbital neighborhood, and is massive enough to be spherical.	The eight planets of the solar system range from rocky inner planets to gas giants in the outer solar system.
22	star	A massive luminous sphere of plasma held together by gravity, generating energy through nuclear fusion.	The star's luminosity and spectral class indicated it was a main-sequence yellow dwarf similar to our sun.
23	light-year	The distance light travels in one year (approximately 9.46 × 10¹² km); a unit of distance, not time.	The nearest star system, Alpha Centauri, is approximately 4.3 light-years from Earth.
24	galaxy	A large system of stars, gas, and dust held together by gravity.	The Milky Way is a spiral galaxy containing an estimated 200–400 billion stars.
25	red shift	The shift of spectral lines toward longer (redder) wavelengths; indicates that an object is moving away.	The redshift observed in distant galaxies provided the first evidence that the universe is expanding.
26	supernova	A massive stellar explosion that occurs at the end of a massive star's life.	The passage describes how supernovae distribute heavy elements throughout the universe.
27	black hole	A region of spacetime where gravity is so strong that nothing, not even light, can escape.	The image captured in 2019 was the first direct observation of a black hole's shadow.
28	asteroid	A rocky body in orbit around the sun, smaller than a planet and mostly found in the asteroid belt.	The Chicxulub impactor, an asteroid approximately 10 km in diameter, contributed to the mass extinction 66 million years ago.
29	comet	An icy body from the outer solar system that develops a tail of gas and dust when near the sun.	Comets are thought to have delivered water and organic molecules to early Earth.
30	lunar cycle	The 29.5-day cycle of the moon's phases as it orbits Earth, from new moon to full moon and back.	Researchers found that breeding behavior in the coral correlated with the lunar cycle.
31	solar system	The sun and all objects gravitationally bound to it, including planets, moons, asteroids, and comets.	The solar system formed approximately 4.6 billion years ago from a collapsing cloud of gas and dust.
32	eclipse	The blocking of light from one celestial body by another; solar (moon blocks sun) or lunar (Earth's shadow on moon).	During a total solar eclipse, astronomers confirmed that starlight bends around the sun, supporting general relativity.
33	convection	Transfer of heat by the movement of fluids (liquids or gases); drives mantle movement and atmospheric circulation.	Convection currents in Earth's mantle drive the movement of tectonic plates.
34	crust (Earth's)	The outermost solid layer of Earth; oceanic crust is thin and dense; continental crust is thick and less dense.	Oceanic crust is continuously created at mid-ocean ridges and destroyed at subduction zones.
35	mantle	The layer of Earth between the crust and core; made of hot, semi-solid rock that flows slowly.	Seismic waves travel more slowly through the mantle than through the crust.
36	core (Earth's)	Earth's innermost layer; the outer core is liquid iron-nickel; the inner core is solid iron-nickel.	Earth's magnetic field is generated by convection of molten iron in the outer core.
37	subduction	The process by which one tectonic plate slides under another into the mantle.	The formation of the Andes mountains resulted from the subduction of the Nazca plate beneath South America.
38	fault	A fracture in Earth's crust along which movement has occurred.	The San Andreas Fault is a transform boundary where two plates slide horizontally past each other.
39	groundwater	Water found underground in pores and cracks in rock and soil; the source of springs and wells.	Contamination of groundwater aquifers posed a significant public health risk in the affected region.
40	glacier	A large, slowly moving mass of ice formed from accumulated snow over many years.	The glacier retreated an average of 12 meters per year between 1980 and 2020.
41	permafrost	Ground that remains frozen year-round; found in polar and alpine regions.	Thawing permafrost releases methane, a potent greenhouse gas, amplifying warming.

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