Gibbs free energy 
noun Chemistry
A thermodynamic quantity equal to the enthalpy (of a system or process) minus the product of the entropy and the absolute temperature. (Symbol: G) The change in Gibbs free energy equals the enthalpy change for the reaction minus the product of the absolute temperature and the change in entropy for the reaction....
- In an equilibrium situation (ice and water at 0 degrees Celsius, or salt and a saturated brine) the Gibbs free energy of both components is equal, and nothing (no melting, freezing, dissolving etc.) happens.
- The amount of mechanical work that can be performed in these states is equal to the change in the Gibbs free energy that occurs in them.
Origin
Named after J. W. Gibbs (see Gibbs, Josiah Willard).