In thermodynamics, the fugacity is a state function of any isothermal system. 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 holds, fugacity is equal to pressure. The ratio formula_1 between fugacity formula_2 and pressure formula_3 is called the fugacity coefficient; therefore, formula_4 for an ideal gas.