Gibbs free energy is a thermodynamics potential that measures the "useful" work obtainable from a thermodynamic system at a constant temperature and pressure. Alternatively, Gibbs energy is the capacity of a system to do non-mechanical work and `DeltaG` measures the non-mechanical work done on it.

The term "free" was attached to Gibbs free energy for systems at constant pressure and temperature to mean "available in the form of useful work. However, an increasing number of books do not include the attachment "free", referring to `G` as simply "Gibbs energy". In `1988` IUPAC meeting the term 'free' was dropped.

lUPAC recommended name : Gibbs energy or Gibbs function.

The Gibbs free energy is mathematically defined as :

`G(p,T) = U + pV -TS`

which is the same as:

`G(p,T) = H -TS`

where: `U` is the internal energy (`SI` unit : joule)
`p` is pressure (`SI` unit : pascal)
`V` is volume (`SI` unit : `m^3` )

`T` is the temperature (`SI` unit : kelvin)
`S` is the entropy (`SI` unit: joule per kelvin)
`H` is the enthalpy (`SI` unit : joule)