Helmholtz Free Energy
The Helmholtz free energy F is one of the four thermodynamic potentials defined by
F=U - TS
Where U is the internal energy, T is the absolute temperature and S is the entropy. The Helmholtz free energy is a measure of the energy required to create a system after the spontaneous energy transfer from the environment has taken place. The quantity TS is energy you can get from the system's environment by heating.
The Helmholtz free energy is constant at constant temperature and volume. It plays a key role in statistical physics.
See also Gibbs Free Energy.