expansion

In physics, the increase in size of a constant mass of substance caused by, for example, increasing its temperature (thermal expansion) or its internal pressure. The expansivity, or coefficient of thermal expansion, of a material is its expansion (per unit volume, area, or length) per degree rise in temperature.

In mechanics, expansion refers to the increase in volume of steam in the cylinder of a steam engine after cutoff, or of gas in the cylinder of an internal-combustion engine after explosion. See also Charles's law.