| A | B |
|---|
| abiotic | In biology, abiotic components are non-living chemical and physical factors in the environment. Despite being non-living, abiotic components can impact evolution. |
| absolute magnitude | the magnitude that a star would have if it were viewed from a distance of 10 parsecs (32.62 light years) from the earth |
| abundant | look up |
| acceleration | rate of change in speed or direction of a moving object |
| Acid | Any various water-soluble compounds having a sour taste and capable of turning litmus red and reacting with a base to form salt |
| Acid rain | look up |
| Actinide | Are radioactive metals that can not be found on earth |
| Adaptation | A characteristic of an organism that helps it to survive and reproduce in a particular environment. |
| Adaptive Radiation | The rise of many new species in a relatively short period of time as a result of a single species invading different habitats. |
| adenine | A purine base, C5H5N5, that is the constituent involved in base pairing with thymine in DNA and with uracil in RNA |
| adhesion | look up |
| Alkali Metal | Any metal in Group 1A of the periodic table |
| alpha particles | a positively charged particle consisting of two protons and two neutrons, emitted in radioactive decay or nuclear fission; the nucleus of a helium atom. |
| amino acid | any of a class of organic compounds that contains at least one amino group, –NH2, and one carboxyl group, –COOH: the alpha-amino acids, RCH(NH2)COOH, are the building blocks from which proteins are constructed. |
| amplitude | the absolute value of the maximum displacement from a zero value during one period of an oscillation |
| Analogous | Structures that have similar functions and specifically similar appearance |
| Analysis | look up |
| Analyze | to examine carefully and in detail so as to identify causes, key factors, or possible results |
| anticodon | a sequence of three nucleotides in a region of transfer RNA that recognizes a complementary coding triplet of nucleotides in messenger RNA during translation by the ribosomes in protein biosynthesis |
| apparent magnitude | a measure of its brightness as seen by an observer on Earth |
| Application | look up |
| asteroids | are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are smaller than planets but larger than meteoroids |
| atmosphere | the mass of air surrounding the Earth |
| atom | the smallest component of an element having the chemical properties of the element, consisting of a nucleus containing combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction |
| atomic number | the number of positive charges or protons in the nucleus of an atom of a given element, and therefore also the number of electrons normally surrounding the nucleus |
| atomic weight | look up |
| Asthenosphere | look up |
| autotrophs | An organism capable of synthesizing its own food from inorganic substances using light or chemical energy. Green plants, algae, and certain bacteria are autotrophs. |
| Average | a quantity, rating, or the like that represents or approximates |
| axial tilt | The inclination of the imaginary line about which a planet rotates to the perpendicular of the planet's orbital plane. |
