The Helmholtz free energy, A (the internal energy of a system minus the product of its entropy and absolute temperature)

(physics, thermodynamics) A measure of the useful work obtainable from a constant temperature, constant volume thermodynamic system; more precisely, the difference between internal energy (of a system) and the product of its absolute temperature and entropy.

(physics, chemistry, thermodynamics) The difference between the enthalpy of a system and the product of its entropy and absolute temperature: a measure of the useful work obtainable from a thermodynamic system at constant temperature and pressure.