A measure of the tendency of a gas to escape or expand. Fugacity is the pressure required at a given temperature to make a non-ideal gas behave as an ideal gas (same Volume, Free Energy, etc.) In thermodynamics, this "effective pressure" is used in place of the real pressure to calculate phase equilibria and changes in energy. Becasue of this use, the fugacity is often written as a product of the pressure with the fugacity coefficient (γ):