aa
a type of lava that has a jagged surface covered with rough volcanic fragments

abdomen
the distinct, usually elongated posterior region of an arthropod's body

abdominal cavity
the hollow part of the body that is between the thorax and the pelvis; contains organs of digestion, excretion, and reproduction

abiotic
describes the nonliving part of the environment, including water, rocks, light, and temperature

abiotic factor
an environmental factor that is not associated with the activities of living organisms

ABO blood group system
a system used to classify human blood by antigens found on the surface of red blood cells

aboral surface
a surface that is opposite or away from the mouth

abrasion
the grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles

abscisic acid
a hormone in plants that helps regulate the growth of buds and the germination of seeds (abbreviation, ABA)

abscission
the separation of leaves or other plant structures by the formation of a layer of cells that develops on the base of the plant structure when the structure is ready to fall

absolute age
the numeric age of an object or event, often stated in years before the present, as established by an absolute-dating process, such as radiometric dating

absolute dating
any method that measures the age of an object or event in years rather than measuring the age relative to another object or event; one such method is radiometric dating

absolute humidity
the mass of water vapor per unit volume of air that contains the water vapor; usually expressed as grams of water vapor per cubic meter of air

absolute magnitude
the brightness that a star would have at a distance of 32.6 light-years from Earth

absolute zero
the temperature at which molecular energy is at a minimum (0 K on the Kelvin scale or –273.16ºC on the Celsius scale)

absorption
in digestion, the process by which the end products of digestion are absorbed by the organism's fluids and cells

absorption
in optics, the transfer of light energy to particles of matter

absorption spectrum
a diagram or graph that indicates the wavelengths of radiant energy that a substance absorbs

abyssal plain
a large, flat, almost level area of the deep-ocean basin

abyssal zone
the benthic environment that extends from beneath the bathyal zone to a depth of up to 6,000 m

acanthodian
an early fish; the earliest known vertebrate to have jaws

acceleration
the rate at which velocity changes over time; an object accelerates if its speed, direction, or both change

accessory pigment
a pigment that absorbs light energy and transfers it to chlorophyll in photosynthesis

acclimation
an organism's change in response to a change in the organism's environment

accuracy
a description of how close a measurement is to the true value of the quantity measured

acetyl-CoA
acetyl coenzyme A, a compound that is synthesized by cells and that plays a major role in metabolism

acid
any compound that increases the number of hydronium ions when dissolved in water; acids turn blue litmus paper red and react with bases and some metals to form salts

acid-base indicator
a substance that changes in color depending on the pH of the solution that the substance is in

acid-ionization constant
the term Ka

acid precipitation
precipitation, such as rain, sleet, or snow, that contains a high concentration of acids, often because of the pollution of the atmosphere

acid rain
precipitation that has a pH below normal and has an unusually high concentration of sulfuric or nitric acids, often as a result of chemical pollution of the air from sources such as automobile exhausts and the burning of fossil fuels

acid shock
the sudden runoff of large amounts of highly acidic water into lakes and streams when snow melts in the spring or when heavy rains follow a drought

acne
an inflammatory disorder of the sebaceous glands that is characterized by skin lesions in the form of pimples, blackheads, and sometimes cysts

acoelomate
an animal that lacks a coelom, or body cavity

acquired trait
a change in an individual organism's structure or function that occurs during the lifetime of the organism, caused by injury, disease, or environmental factors, and that is not inherited by future generations

ACTH
adrenocorticotropic hormone, a hormone that stimulates the adrenal cortex

actin
a protein responsible for the contraction and relaxation of muscle

actinide
any of the elements of the actinide series, which have atomic numbers from 89 (actinium, Ac) through 103 (lawrencium, Lr)

actinomycete
a member of a class of bacteria that form branched filaments and that can be found in soil, in water, or as pathogens in animals and plants

action potential
a sudden change in the polarity of the membrane of a neuron, gland cell, or muscle fiber that facilitates the transmission of electrical impulses

activated complex
a molecule in an unstable state intermediate to the reactants and the products in the chemical reaction

activation
in genetics, the initiation of transcription of a particular gene

activation energy
the minimum amount of energy required to start a chemical reaction

active site
the site on an enzyme that attaches to a substrate

active solar heating
the gathering of solar energy by collectors that are used to heat water or heat a building

active transport
the movement of chemical substances, usually across the cell membrane, against a concentration gradient; requires cells to use energy

activity series
a series of elements that have similar properties and that are arranged in descending order of chemical activity; examples of activity series include metals and halogens

actual yield
the measured amount of a product of a reaction

acyclovir
an antiviral drug

adapt
in populations, to change genetically over generations in response to the natural environment such that the population has an increased ability to survive and reproduce

adaptation
the process of becoming adapted to an environment; an anatomical, physiological, or behavioral change that improves a population's ability to survive

adaptive radiation
an evolutionary pattern in which many species evolve from a single ancestral species

addiction
a physiological or psychological dependence on a substance, such as alcohol or drugs

addition polymer
a polymer in which the formula of the repeating structural unit is identical to the molecular formula of the monomer

addition reaction
a reaction in which an atom or molecule is added to an unsaturated molecule

adductor muscle
the thick muscle that joins the two valves in mollusks and that causes the shell to open

adenine
one of the four bases that combine with sugar and phosphate to form a nucleotide subunit of DNA; adenine pairs with thymine

ADH
antidiuretic hormone, a hormone that regulates the resorption of water from the kidney

adhesion
the attractive force between two bodies of different substances that are in contact with each other

adiabatic
the process that occurs when an air mass rises and expands without exchanging heat with its surroundings

adiabatic cooling
the process by which the temperature of an air mass decreases as the air mass rises and expands

adiabatic process
a thermodynamic process in which no heat transfer between the system and its environment takes place

ADP
adenosine diphosphate, an organic molecule that is involved in energy metabolism; composed of a nitrogenous base, a sugar, and two phosphate groups

adrenal gland
one of the two endocrine glands located above each kidney

advection fog
condensation of water vapor that results from the cooling of warm, moist air as the air moves across a cold surface

advective cooling
the process by which the temperature of an air mass decreases as the air mass moves over a cold surface

adventitious root
a root that occurs in an unusual position, such as on stems or leaves

aerial photograph
a photograph of the Earth's surface that is taken from the air

aerobic
describes a process that requires oxygen

aerobic exercise
physical exercise intended to increase the activity of the heart and lungs to promote the body's use of oxygen

afferent neuron
a neuron that conducts impulses toward the central nervous system

aflatoxin
a toxic fungal poison that causes liver cancer; found as contaminants in peanuts and corn; produced by a species of Aspergillus

afterbirth
the remains of the placenta and the membranes, which are expelled from the mother's body following birth

aftershock
an earthquake that follows the main shock of a large earthquake

agar
a gel-like base for culturing microbes; extracted from certain red algae

Agent Orange
a toxic substance used to cause plants to lose their leaves

age structure
the classification of members of a population into groups according to age or the distribution of members of a population in terms of age groups

aggregation
a grouping of cells or other organisms

agnathan
a member of a class of primitive, jawless fishes

agricultural revolution
the change from a hunting and gathering society to an agricultural society that began about 10,000 years ago

agriculture
the raising of crops and livestock for food or for other products that are useful to humans

AIDS
acquired immune deficiency syndrome, a disease caused by HIV, an infection that results in an ineffective immune system

air mass
a large body of air throughout which temperature and moisture content are similar

air pollution
the contamination of the atmosphere by the introduction of pollutants from human and natural sources

air pressure
the measure of the force with which air molecules push on a surface

albedo
the fraction of solar radiation that is reflected off the surface of an object

albumen
the white of an egg, composed primarily of the protein albumin

alcohol
an organic compound that contains one or more hydroxyl groups attached to carbon atoms

alcoholic fermentation
the anaerobic process by which yeasts and other microorganisms break down sugars to form carbon dioxide and ethanol

alcoholic hepatitis
liver inflammation resulting from alcoholism

alcoholism
a chronic disorder in which a person repeatedly drinks alcoholic beverages in an amount that interferes with the person's health and activities; the term implies a certain degree of addiction or dependence

aldehyde
an organic compound that contains the carbonyl group, —CHO

aldosterone
a hormone that helps maintain water and salt balance

algae
eukaryotic organisms that convert the sun’s energy into food through photosynthesis but that do not have roots, stems, or leaves (singular, alga)

alginate
a salt of alginic acid

alkali metal
one of the elements of Group 1 of the periodic table (lithium, sodium, potassium, rubidium, cesium, and francium)

alkaline
a solution that has a pH greater than 7 because of the dissociation of a base in water to produce hydroxide ions

alkaline-earth metal
one of the elements of Group 2 of the periodic table (beryllium, magnesium, calcium, strontium, barium, and radium)

alkane
a hydrocarbon characterized by a straight or branched carbon chain that contains only single bonds

alkene
a hydrocarbon that contains one or more double bonds

alkyl group
a group of atoms that forms when one hydrogen atom is removed from an alkane molecule

alkyl halide
a compound formed from an alkyl group and a halogen (fluorine, chlorine, bromine, or iodine)

alkyne
a hydrocarbon that contains one or more triple bonds

allantois
a membranous sac that acts as an organ of respiration and nutrition for the embryo; in humans, its blood vessels become the blood vessels of the umbilical cord

allele
one of the alternative forms of a gene that governs a characteristic, such as hair color

allele frequency
the proportion of gene copies in a population that are a given allele, expressed as a percentage

allergy
a physical response to an antigen, which can be a common substance that produces little or no response in the general population

alloy
a solid or liquid mixture of two or more metals

alluvial fan
a fan-shaped mass of rock material deposited by a stream when the slope of the land decreases sharply; for example, alluvial fans form when streams flow from mountains to flat land

alluvium
rock and soil carried and deposited by streams

alpha decay
the release of an alpha particle from a nucleus

alpha particle
a positively charged atom that is released in the disintegration of radioactive elements and that consists of two protons and two neutrons

alpine glacier
a narrow, wedge-shaped mass of ice that forms in a mountainous region and that is confined to a small area by surrounding topography; examples include valley glaciers, cirque glaciers, and piedmont glaciers

alternating current
an electric current that changes direction at regular intervals (abbreviation, AC)

alternation of generations
the alternation of sexual reproduction and asexual reproduction in certain plants and animals

alternative energy
energy that does not come from fossil fuels and that is still in development

altitude
in astronomy, the angle between an object in the sky and the horizon

altitude
the height of an object above a reference point, such as sea level or the Earth's surface

altricial
describes organisms that are not very developed when born and that require care or nursing

altruistic behavior
self-sacrificing behavior that benefits another individual

alveolus
any of the tiny air cells of the lungs where oxygen and carbon dioxide are exchanged

amber
a hardened tree resin (a thick, saplike substance) in which fossils may be trapped and preserved

amine
an organic compound that can be considered to be a derivative of ammonia

amino acid
any one of 20 different organic molecules that contain a carboxyl and an amino group and that combine to form proteins

amino acid–based hormone
a hormone that is made up of simple amino acids, peptides, or proteins

ammonia
a colorless gas that has an alkaline reaction in water; forms in nature as an end product of animal metabolism (formula, NH3)

ammonification
the formation of ammonia compounds in the soil by the action of bacteria on decaying matter

amniocentesis
a procedure used in fetal diagnosis in which amniotic fluid is removed from the uterus of the pregnant woman

amnion
the membrane that contains a developing embryo and its surrounding fluid

amniotic egg
a type of egg that is produced by reptiles, birds, and egg-laying mammals and that contains a large amount of yolk; usually surrounded by a leathery or hard shell within which the embryo and its embryonic membranes develop

amniotic sac
the sac formed by the amnion

amoebic dysentery
a disease that is caused by an amoeba and that causes inflammation of the intestine

amoebocyte
in sponges and other invertebrates, an amoeba-like cell that moves through the body fluids, removes wastes, and participates in other processes

amoeboid movement
amoeba-like movement; the movement of a cell by the formation of pseudopods

amorphous solid
a solid in which the particles are not arranged with periodicity or order

amphoteric
describes a substance, such as water, that has the properties of an acid and the properties of a base

amplexus
the copulatory embrace of amphibians

amplitude
the maximum distance that the particles of a wave’s medium vibrate from their rest position

amygdala
a structure or body in the form of an almond; an almond-shaped structure located in the brain

amylase
an enzyme that breaks down starches into sugars

anaerobic
describes a process that does not require oxygen

anal fin
a single fin usually located behind the anus of a fish

analogous
in comparisons of different organisms, describes features that are similar in function and appearance but not in structure or origin

analog signal
a signal whose properties, such as amplitude and frequency, can change continuously in a given range

anal pore
in protozoans, an opening from which wastes are eliminated

anaphase
a phase of mitosis and meiosis in which the chromosomes separate

androgen
a hormone that is secreted by the testes and that controls secondary male characteristics

anemia
a condition in which the oxygen-carrying ability of red blood cells is reduced and the production of red blood cells decreases

anemometer
an instrument used to measure wind speed

angiosperm
a flowering plant that produces seeds within a fruit

angle of incidence
the angle between a ray that strikes a surface and the perpendicular to that surface at the point of contact

angle of reflection
the angle formed by the line perpendicular to a surface and the direction in which a reflected ray moves

angular acceleration
the time rate of change of angular velocity expressed in radians per second per second

angular displacement
a change in angular position, described by a rotation around an axis

angular momentum
for a rotating object, the product of the object’s position and linear momentum at the moment the object passes a given point

angular momentum quantum number
the quantum number that indicates the shape of an orbital

angular speed
the rate at which a body rotates about an axis, usually expressed in radians per second

angular unconformity
an unconformity, or break in the geologic record, created when younger sediment is deposited on top of the eroded surface of older rocks that have been tilted or folded

Animalia
the classification kingdom containing complex, multicellular organisms that lack cell walls, are usually able to move around, and possess specialized sense organs that help them quickly respond to their environment

anion
an ion that has a negative charge

annelid
a worm that has ringlike segments, a true coelom, and a closed vascular system, such as an earthworm or a leech

annual
a plant that completes its life cycle, reproduces, and dies within one growing season

annual ring
in secondary xylem (wood), the growth ring formed in one season

anode
the electrode on whose surface oxidation takes place; anions migrate toward the anode, and electrons leave the system from the anode

antacid
a weak base that neutralizes stomach acid

antagonistic hormone
a hormone that counteracts the effect of another hormone

antenna
a feeler that is on the head of an invertebrate, such as a crustacean or an insect, and that senses touch, taste, or smell

antennule
in crustaceans, a second pair of antennae, usually smaller than the first

anterior
the front part of a body or structure

anther
in flowering plants, the tip of a stamen, which contains the pollen sacs where grains form

antheridium
a reproductive structure that produces male sex cells in flowerless and seedless plants

anthracite
one of the hardest forms of coal

anthropoid
an ape that resembles a human; examples include chimpanzees, gorillas, and some ancient apes

anthropoid primate
one of a subgroup of primates that includes monkeys, apes, and humans

antibiotic
a substance that can inhibit the growth of or kill some microorganisms

antibody
a protein that reacts to a specific antigen or that inactivates or destroys toxins

anticline
an arch-shaped fold in rock layers in which the oldest layer is in the center of the fold

anticodon
a region of tRNA that consists of three bases complementary to the codon of mRNA

anticyclone
the rotation of air around a high-pressure center in the direction opposite to Earth's rotation

antigen
a substance that stimulates an immune response

antigen shifting
the production of new antigens by a virus as it mutates over time

antinode
a point in a standing wave, halfway between two nodes; it indicates a position of maximum intensity

antiviral drug
a drug that destroys viruses or prevents their growth or replication

anus
the posterior opening of the digestive tract

aorta
the main artery in the body; it carries blood from the left ventricle to systemic circulation

aortic arch
the bend of the aorta between its ascending and descending portions

aortic valve
the valve that regulates the flow of blood from the left ventricle to the aorta

aphelion
the point in the orbit of a planet at which the planet is farthest from the sun

aphotic zone
the deeper parts of the ocean, where there is so little light that plants cannot carry out photosynthesis

apical dominance
the inhibition of lateral bud growth on the stem of a plant by auxin produced in the terminal bud

apical meristem
the growing region at the tips of stems and roots in plants

apical organ
a ciliated plate located at the back of the larva of an annelid

apogee
in the orbit of a satellite, the point at which the satellite is farthest from Earth

apparent magnitude
the brightness of a star as seen from Earth

appendage
a structure that extends from the main body, such as a limb, tentacle, fin, or wing

appendicular skeleton
the bones of the arms and legs

applied science
the use of science to solve practical problems

aquaculture
the raising of aquatic plants and animals for human use or consumption

aqueous solution
a solution in which water is the solvent

aquifer
a body of rock or sediment that stores groundwater and allows the flow of groundwater

arable land
farmland that can be used to grow crops

arachnid
an arthropod that has eight legs and no wings or antennae; a spider, scorpion, mite, or tick

arachnoid membrane
the middle membrane of the three sacs that surround the brain and spinal cord

Archaea
in a modern taxonomic system, a domain made up of prokaryotes (most of which are known to live in extreme environments) that are distinguished from other prokaryotes by differences in their genetics and in the makeup of their cell wall; this domain aligns with the traditional kingdom Archaebacteria

archaea
prokaryotes (most of which are known to live in extreme environments) that are distinguished from other prokaryotes by differences in their genetics and in the makeup of their cell wall; members of the domain Archaea (singular, archaeon)

Archaebacteria
a kingdom made up of prokaryotes (most of which are known to live in extreme environments) that are distinguished from other prokaryotes by differences in their genetics and in the makeup of their cell wall; currently, biologists prefer to classify members of this kingdom into the domain Archaea

archaebacteria
prokaryotic organisms that are distinguished from other prokaryotes by differences in their genetics and in the makeup of their cell wall; currently, biologists prefer to call these organisms archaea and to classify them into the domain Archaea (singular, archaebacterium)

archegonium
a female reproductive structure of small, nonvascular plants that produces a single egg and in which fertilization and development take place

archenteron
the primitive gastric cavity of an embryo

Archimedes' principle
the principle that states that the buoyant force on an object in a fluid is an upward force equal to the weight of the volume of fluid that the object displaces

area
a measure of the size of a surface or a region

arête
a sharp, jagged ridge that forms between cirques

aromatic hydrocarbon
a member of the class of hydrocarbons (of which benzene is the first member) that consists of assemblages of cyclic conjugated carbon atoms and that is characterized by large resonance energies

Arrhenius acid
a substance that increases the concentration of hydronium ions in aqueous solution

Arrhenius base
a substance that increases the concentration of hydroxide ions in aqueous solution

arteriole
a small artery that gives rise to capillaries

artery
a blood vessel that carries blood away from the heart to the body's organs

artesian formation
a sloping layer of permeable rock sandwiched between two layers of impermeable rock and exposed at the surface

artesian spring
a spring whose water flows from a crack in the bed over the artesian aquifer

artesian well
a well whose water comes from an artesian aquifer

arthropod
a member of the phylum Arthropoda, which includes invertebrate animals such as insects, crustaceans, and arachnids; characterized by having segmented bodies and paired appendages

artificial eutrophication
a process that increases the amount of nutrients in a body of water through human activities, such as waste disposal and land drainage

artificial satellite
any human-made object placed in orbit around a body in space

artificial selection
the selective breeding of organisms (by humans) for specific desirable characteristics

artificial transmutation
the transformation of atoms of one element into atoms of another element as a result of a nuclear reaction, such as bombardment with neutrons

asbestos
any of six silicate minerals that form bundles of minute fibers that are heat resistant, flexible, and durable

ascocarp
the reproductive portion of an ascomycete

ascogonium
the female sexual organ or cell in ascomycetes

ascospore
a spore produced in an ascus by ascomycetes

ascus
the spore sac where ascomycetes produce ascospores

asexual reproduction
reproduction that does not involve the union of gametes and in which a single parent produces offspring that are genetically identical to the parent

aspirin
the common name of acetylsalicylic acid, a pain reliever

assortative mating
sexual reproduction in which males and females do not breed randomly

asteroid
a small, rocky object that orbits the sun; most asteroids are located in a band between the orbits of Mars and Jupiter

asteroid belt
the region of the solar system that is between the orbits of Mars and Jupiter and in which most asteroids orbit

asthenosphere
the solid, plastic layer of the mantle beneath the lithosphere; made of mantle rock that flows very slowly, which allows tectonic plates to move on top of it

astronomical unit
the average distance between the Earth and the sun; approximately 150 million kilometers (symbol, AU)

astronomy
the scientific study of the universe

asymmetrical
irregular in shape; without symmetry

asymmetry
a lack of symmetry

atherosclerosis
a disease characterized by the buildup of fatty materials on the interior walls of the arteries

atmosphere
a mixture of gases that surrounds a planet or moon

atmosphere of pressure
the pressure of the Earth's atmosphere at sea level; exactly equivalent to 760 mm Hg

atmospheric pressure
the force per unit area that is exerted on a surface by the weight of the atmosphere

atmospheric transmission
the passage of an electromagnetic wave signal through the atmosphere between a transmitter and a receiver

atoll
a nearly circular coral reef surrounding a lagoon

atom
the smallest unit of an element that maintains the chemical properties of that element

atomic mass
the mass of an atom expressed in atomic mass units

atomic mass unit
a unit of mass that describes the mass of an atom or molecule; it is exactly 1/12 of the mass of a carbon atom with mass number 12 (symbol, amu)

atomic number
the number of protons in the nucleus of an atom; the atomic number is the same for all atoms of an element

atomic radius
one-half of the distance between the center of identical atoms that are not bonded together

ATP
adenosine triphosphate, an organic molecule that acts as the main energy source for cell processes; composed of a nitrogenous base, a sugar, and three phosphate groups

ATP synthetase
an enzyme that catalyzes the synthesis of ATP

atriopore
an opening from the atrial cavity to the exterior in certain fish

atrioventricular node
heart tissues that generate the activity that produces heartbeat (abbreviation, AV node)

atrioventricular valve
a one-way valve that separates each atrium from the ventricle beneath it (abbreviation, AV valve)

atrium
a chamber that receives blood that is returning to the heart

attenuated strain
a virus strain that has less potency and that is used in vaccines to produce an immune response

auditory canal
the tube through which air enters the ear

Aufbau principle
the principle that states that the structure of each successive element is obtained by adding one proton to the nucleus of the atom and one electron to the lowest-energy orbital that is available

aurora
colored light produced by charged particles from the solar wind and from the magnetosphere that react with and excite the oxygen and nitrogen of Earth’s upper atmosphere; usually seen in the sky near Earth’s magnetic poles

australopithecine
an early hominid that may have lived more than 3.6 million years ago

autoimmune disease
a disease in which the immune system attacks the organism's own cells

autonomic nervous system
the part of the nervous system that controls involuntary actions

autooxidation
a spontaneous, self-catalyzed oxidation that occurs in the presence of air and that is initiated by factors such as heat or light

autosome
any chromosome that is not a sex chromosome

autotomy
the ability of an organism to drop a body part and, usually, to regenerate a new one

autotroph
an organism that produces its own nutrients from inorganic substances or from the environment instead of consuming other organisms

autumnal equinox
the moment when the sun passes directly above the equator from north to south; day and night are of equal length on the day that the autumnal equinox occurs

auxin
a plant hormone that regulates cell elongation

average atomic mass
the weighted average of the masses of all naturally occurring isotopes of an element

average velocity
the total displacement divided by the time interval during which the displacement occurred

Avogadro's law
the law that states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

Avogadro's number
6.02 ¥ 1023, the number of atoms or molecules in 1 mol

axial skeleton
the bones of the skull and vertebral column

axis
an imaginary straight line running through the Earth from pole to pole

axon
an elongated extension of a neuron that carries impulses away from the cell body

azimuthal projection
a map projection that is made by moving the surface features of the globe onto a plane

AZT
azidothymidine, a drug that blocks DNA replication and is useful in the treatment of AIDS