Gibbs Free Energy
The Gibbs free energy is constant if the temperature and pressure are constant. These are the conditions under which phase transitions (melting, boiling) take place, and are also relevant to chemical equilibrium.
The Gibbs Free Energy is given by:
G = U + P V - T S
Where U is the internal energy, P the pressure, V the volume, T the temperature in Kelvin, S the entropy.
The Gibbs Free Energy can be though of as the maximum energy available from a chemical reaction.