1. A form of energy transferred between systems, existing only in the process of transfer. 2. Same as enthalpy. Heat, used as a noun, is confusing and controversial in its scientific meaning. The differential of heat is considered imperfect in that its value depends on the process applied. In the thermodynamic definitions in this glossary, heat is avoided as a noun or adjective except where required by established use. The process of heating is, however, defined as the net absorption of internal energy by a system.