free energy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A thermodynamic quantity that is the difference between the internal energy of a system and the product of its absolute temperature and entropy; the capacity of a system to do work, as in an exothermic chemical reaction.
• noun A thermodynamic quantity that is the difference between the enthalpy and the product of the absolute temperature and entropy of a system.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The difference between the internal energy of a system and the product of its entropy and absolute temperature
• noun physics The Gibbs free energy

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (physics) a thermodynamic quantity equivalent to the capacity of a physical system to do work; the units of energy are joules or ergs