In thermodynamics, an expression representing
part of the energy content of a system that
is available to do external work, also known
as the free energy G. In an equilibrium
system at constant temperature and pressure,
G = H - TS, where H is the enthalpy (heat
constant), T the temperature, and S the
entropy (decrease in energy availability).
The function was named after US physicist
Josiah Willard Gibbs.