| A | B |
|---|
| Abiotic factor | a nonliving physical or chemical part of an ecosystem |
| Abrasion | the process of wearing something down by friction |
| Absolute age | the actual age in years of an event or object |
| Absorption | the disappearance of a wave into a medium; when a wave is absorbed |
| Acceleration | the rate of change in velocity |
| Acid | a substance that can donate a proton to another substance and has a pH below 7 |
| Acid rain | rain that has become more acidic than normal due to pollution |
| Acoustics | the scientific study of sound; the behavior of sound waves inside a space |
| Active transport | the process of using energy to move materials through a membrane |
| Adaptation | a characteristic that helps an organism survive in its habitat |
| Addiction | a physical or psychological need for a habit-forming substance |
| Aftershock | a smaller earthquake that follows a more powerful earthquake in the same area |
| Adolescence | the stage of life from the time a human body begins to mature sexually to adulthood |
| Adulthood | the stage of life that begins once a human body completes its growth and reaches sexual maturity |
| Air mass | a large body of air that has a uniform temperature and humidity throughout |
| Air pollution | a large amount of air that has nearly the same temperature and humidity at different locations at the same altitude |
| Air pressure | the force of air molecules pushing on an area |
| Air resistance | the fluid friction due to air |
| Algae | Protists that live mostly in water and use sunlight as a source of energy |
| Allele | an alternate form of a gene for a specific trait or gene product |
| Alloy | a solid mixture composed of a metal and one or more other substances |
| Alluvial fan | a fan-shaped deposit of sediment at the base of a slope |
| Alternating current (AC) | electric current that reverses direction at regular intervals |
| Altitude | the height above sea level of a place |
| Ampere (amp) | the unit of measurement of electric current |
| Amphibian | a cold-blooded vertebrate animal that lives in water and breathes with gills when it is young; as an adult |
| Amplification | the strengthening of an electrical signal |
| Amplitude | the height of the crest or the depth of the trough of a wave measured from the undisturbed surface |
| Analog | represented by a continuous but varying quantity |
| Ancestor | a distant or early form of an organism from which later forms descend |
| Angiosperm | a plant that has flowers and produces seeds enclosed in fruit |
| Animalia | part of a classification system that divides all living things into six kingdoms; Kingdom Animalia includes multi-cellular organisms |
| Antibiotic | a medicine that can block the growth and reproduction of bacteria |
| Antibody | a protein produced by some white blood cells to attack specific foreign materials |
| Antigen | a marker that a pathogen carries and that stimulates the production of antibodies |
| Appendicular skeleton | the bones of the skeleton that function to allow movement |
| Aquifer | an underground layer of permeable rock that contains water |
| Archaea | part of a classification system that divides all living things into six kingdoms; Kingdom Archaea includes microscopic organisms with a distinctive cell structure that allows them to live in extreme environments |
| Artery | a blood vessel with strong walls that carries blood away from the heart |
| Arthropod | an invertebrate animal with an exoskeleton |
| Asexual reproduction | the form of reproduction that involves only one parent producing offspring that are genetically identical to the parent |
| Asteroid | a small |
| Asthenosphere | the layer in Earth’s upper mantle and directly under the lithosphere in which rock is soft and weak because it is close to melting |
| Astronomical unit (AU) | Earth’s average distance from the sun |
| Atmosphere | the outer layer of gases of al large body in space |
| Atom | the smallest particle of an element that has the properties of that element |
| Atomic mass | the average mass of the atoms of an element |
| Atomic mass number | the total number or protons and neutrons in an atom’s nucleus |
| Autotroph | an organism that captures energy from sunlight and uses it to produce energy-rich carbon compound |
| Axial skeleton | the central part of the skeleton |
| Axis of rotation | an imaginary line about which a turning body rotates |
