exergy is the maximum amount of work that can be extracted from a material by reversible processes as it approaches thermodynamic equilibrium with its surroundings. Exergy is therefore a quantity that is not definable in absolute terms. It can only be defined in terms of a reference state, namely the environment. But exergy can be calculated for any material with reference to whatever environmental medium that material would be likely to reach thermodynamic equilibrium with, namely the atmosphere, the ocean or the surface layer of the earth’s crust (topsoil or subsoil). Thus gases tend to equilibrate with the atmosphere, liquids or soluble solids with the oceans, insoluble solids with the land. ( Two Paradigms of Production and Growth, Professor Robert U. Ayres, Dr. Benjamin Warr, Center for the Management of Environmental Resources (CMER), INSEAD, France)

The energy of a system which is usable as work. Zero entropy energy.

the maximum amount of energy that under given (ambient) thermodynamic conditions can be converted into any other form or energy; also known as availability or work potential