free energy

Short definition from Wiktionary

Noun

1. 1
   The energy available in a particular chemical system to do useful work. In more detail: Helmholtz free energy, A: the internal energy of a system minus the product of its entropy and absolute temperature.
2. 2
   The energy available in a particular chemical system to do useful work. In more detail: Gibbs free energy, G: the enthalpy of a system minus the product of its entropy and absolute temperature.
3. 3
   Used other than figuratively or idiomatically: see free, energy.