The heat evolved or absorbed during the formation of one mole of a substance from its component elements WordNet 3.0 & Open English WordNet

In chemistry and thermodynamics, the standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements in their reference state, with all substances in their standard states. Wikipedia

change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states, and at a pressure of 1 bar Wikidata