In thermodynamics, the fugacity is a state function of matter at fixed temperature. The fugacity, which has units of pressure, represents the tendency of a fluid to escape or expand isothermally. For gases at low pressures where the ideal gas law is a good approximation, fugacity is nearly equal to pressure. The ratio between fugacity and pressure is called the fugacity coefficient. For an ideal gas, .

For a given temperature , the fugacity is defined by the following differential relation:

where is the Gibbs free energy, is the molar gas constant, and is the fluid's volume. Thus, for any two mutually-isothermal physical states, represented by subscripts 1 and 2, the ratio of the two fugacities is as follows: