The thermodynamic quantity defined as H = U + PV is enthalpy. Here, H is enthalpy, U is internal energy, P is pressure, and V is volume.
The PV term represents the pressure–volume energy associated with making room for a system against its surroundings. Enthalpy is therefore especially useful for processes occurring at constant pressure, such as many reactions carried out in an open laboratory atmosphere. Although the total enthalpy of a system cannot usually be measured directly, changes in enthalpy can be measured and compared.
At constant pressure, when there is no additional electrical or mechanical work, an enthalpy change can equal the heat transferred to or from the system. An exothermic reaction has negative ΔH because it releases heat; an endothermic reaction has positive ΔH because it absorbs heat. Enthalpy is a state function, so its change depends only on the initial and final states, not on the route between them. It is commonly confused with internal energy, but the PV term is the key distinction.