Expansion is the increase in length, area, or volume of a body as a result of changing conditions, normally increasing temperature or decreasing pressure, the latter being more important for gases. Contraction is the reverse process. In most solids and liquids, increasing the temperature increases the random thermal motion of their atoms, which tend to move apart, i.e., expansion occurs.
The amount of expansion is usually expressed as a coefficient of expansion – the fractional change in length, area, or volume per unit temperature change – and is specific for a given material. A solid has three coefficients of expansion – linear, superficial, and volume. For a gas, the coefficient of expansion is the rate of change of volume with temperature (at constant pressure), or of volume with pressure (at constant temperature). For a solid, the coefficient of expansion is usually small; for a gas it is much larger. See also Charles' law.