absolute temperature: Temperature reckoned on a scale that places zero at about —273 degrees on the Celsius scale.
acceleration: The rate of change of velocity with time; measured in meters per second per second, feet per second per second, etc.
acceleration of gravity: The rate of change of velocity with time due to gravitational attraction; on Earth equal to about 32.2 feet per second per second. Represented by the symbol g.
adiabatic system: A system insulated thermally from its surroundings.
algebra: A branch of mathematics that generalizes arithmetic by representing numbers with symbols.
alpha particles (or rays): Helium ions originating in radioactive decay.
amplitude: In quantum mechanics, a quantity calculated for an event and squared to obtain the probability for occurrence of the event.
angstrom: A very small distance unit, equal to 1CT8 centimeter.
anion: A negatively charged ion.
anode: In electrochemistry, the positive electrode of an electrolysis cell, toward which negative ions (anions) are attracted.
antielectron: A positive electron or positron.
antiparticle: A particle that is like its corresponding particle except that it has a charge and certain other properties opposite to those of the particle. When a particle and its corresponding antiparticle meet, they annihilate each other, leaving only energy. All particles of matter have their anti counterparts.
astronomy: The study of stars, galaxies, and other celestial objects, through observations with telescopes and associated instruments.
astrophysics: The theoretical study of the physical nature of stars, galaxies, and other celestial objects.
atomic number: A number assigned to each chemical element that determines the element's place in the periodic table; also equal to the charge on the element's atomic nucleus in units of the proton charge.
atomic weight: The mass of an atom relative to the mass of a hydrogen atom taken to be about 1 (actually, 1.008).
Avogadro's number: The number of molecules of hydrogen in about 2 grams (actually, 2.016 grams) of hydrogen.
baryon: A heavy hadron composed of three quarks; examples are protons and neutrons.
base: Of a logarithm, the number that is raised to a power equal to the logarithm.
beta particles (or rays): Electrons originating in radioactive decay.
blackbody: An object that emits its own radiation when heated, but does not reflect incident radiation.