absolute humidity: The mass of water vapour in a unit volume of air.
absolute zero: The lowest temperature possible. A theoretical limit that represents a state where an object has its constituent particles at rest. Absolute zero refers to 0 K (Kelvin scale) or -273 °C (Celsius scale).
abyssal plain: The flat surface of the sea floor.
AC electricity: Electricity that operates on alternating voltages and alternating currents.
acid: A substance capable of donating a hydrogen ion (proton).
acidity: A measure of the concentration of hydrogen ions (protons) present in a solution.
acid rain: Acid produced in the atmosphere as a result of air pollution.
action force: The force applied to another object. In return there is an equal and opposite reaction force.
activation energy: The amount of energy required to break a chemical bond.
adsorption: The attraction of one substance or molecule to the surface of another.
aftershock: An earthquake caused by adjustments along a fault following a larger earthquake.
air foil: The shape of plane wings. The shape is such as to be more curved on one side.
air pressure: The pressure of the air as a result of the collisions of atmospheric particles bombarding the surface of a material. Pressure is the amount of force applied per unit of surface area.
air resistance: Fluid friction that tends to slow an object through the air particles pushing on it.
albedo: The fraction of light reflected from an object. For example, the Moon has an albedo of 0.07 which means that 7 percent of the light incident on the Moon is reflected.
allotropes: Different forms of the one element. For example, diamond and graphite are allotropes of carbon.
alloy: A homogeneous mixture of two different metals.
alternating voltage or current: This is where the current in an electric circuit is constantly changing direction. This can be achieved by constantly changing the polarity of the terminals of the battery or generator. That is, one moment the terminal of a battery will be positive and the next moment it will change to negative. The other terminal changes accordingly.
altitude: The angular elevation of an object in the sky.
ammeter: A device that measures the current in an electric circuit. It must be placed directly in the conducting path.
amorphous substance: A substance without shape or form, the particles of which are arranged in an irregular manner. Glass is an example of an amorphous substance.
ampere: The unit of electric current.
amphiprotic substance: A substance that can act as an acid or a base depending on its environment.
aneroid barometer: A barometer that does not use liquid.
anion: A negatively charged ion.
annealing: The process where a metal is heated and cooled slowly to make the metal stronger.
annular eclipse: A type of solar eclipse in which a ring (annulus) of the outer part of the Sun (solar photosphere) remains visible around the Moon.
anode: The electrode at which oxidation occurs. It is negatively charged as electrons leave from here.
anticline: An upward fold.
antimatter: A type of matter in which each particle (antiproton, antineutron, etc.) is opposite in charge and other properties to the corresponding particles (protons, neutrons, etc.) of matter.
aphanitic: A fine-grained texture in igneous rocks in which the individual mineral grains are too small to be seen with the naked eye.
aphelion: For an orbit around the Sun, the farthest point from the Sun.
apogee: The farthest point in an elliptical orbit.
asteroid: A rocky object, smaller than a planet, in orbit around the Sun.
asteroid belt: A region of the solar system, between the orbits of Mars and Jupiter where most of the asteroids orbit.
asthenosphere: The part of the mantle below the lithosphere; it behaves plastically and flows.
astrology: A non-scientific system based on superstition that purports to explain or predict human actions by study of celestial positions.
astronomical unit (AU): The average distance from Earth to the Sun. 1 AU = 150 million kilometres.
atom: The smallest part of an element that can take part in a chemical reaction. An atom is a very small particle of matter that consists of a nucleus containing protons and neutrons. Electrons surround the nucleus. The atom contains mostly space. All matter is composed of atoms.
atomic number: The number of protons in the nucleus of an atom.
autumnal equinox: One of the two locations in the sky where the ecliptic crosses the celestial equator that the Sun passes each year when moving from northern to southern declinations.
azimuth: The angular distance, around the horizon from the northern direction, usually expressed in angular measure from 0° for an object directly north to 180° for an object due south. Together, the altitude and azimuth coordinates define the direction to a celestial object.