enthalpy science definition

A partial measure of the internal energy of a system. Enthalpy cannot be directly measured, but changes in it can be. If an outside pressure on a system is held constant, a change in enthalpy entails a change in the system's internal energy, plus a change in the system's volume (meaning the system exchanges energy with the outside world). For example, in endothermic  chemical reactions, the change in enthalpy is the amount of energy absorbed by the reaction; in exothermic  reactions, it is the amount given off. See also thermodynamics.