enthalpy

n. sum of the internal heat in a system plus the product of the system's volume and external pressure (Thermodynamics)

In thermodynamics and molecular chemistry, the enthalpy or heat content (denoted as H or ΔH, or rarely as χ) is a quotient or description of thermodynamic potential of a system, which can be used to calculate the "useful" work obtainable from a closed thermodynamic system under constant pressure. The term enthalpy is composed of the prefix en-, meaning to "put into", plus the Greek word -thalpein, meaning "to heat", although the original definition is thought to have stemmed from the word, "enthalpos" (ἐνθάλπος). It is often calculated as a differential sum, describing the changes within exo- and endothermic reactions, which minimize at equilibrium.
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(H) Compare with heat .Enthalpy (H) is defined so that changes in enthalpy ( H) are equal to the heat absorbed or released by a process running at constant pressure. While changes in enthalpy can be measured using calorimetry, absolute values of enthalpy usually cannot be determined. Enthalpy is formally defined as H = U + PV, where U is the internal energy, P is the pressure, and V is the volume.

Noun
1. (thermodynamics) a thermodynamic quantity equal to the internal energy of a system plus the product of its volume and pressure; "enthalpy is the amount of energy in a system capable of doing mechanical work"
(synonym) heat content, total heat, H
(hypernym) physical property
(classification) thermodynamics