**Definition:**Gibbs free energy is a thermodynamic property that was defined in 1876 by Josiah Willard Gibbs to predict whether a process will occur spontaneously at constant temperature and pressure. Gibbs free energy G is defined as G = H - TS where H, T and S are the enthalpy, temperature, and entropy.

Changes in the Gibbs free energy G correspond to changes in free energy for processes at constant temperature and pressure. The change in Gibbs free energy change is the maximum nonexpansion work obtainable under these conditions. ΔG is negative for spontaneous processes, positive for nonspontaneous processes and zero for processes at equilibrium.

**Also Known As:**(G), Gibbs' free energy