The letter that symbolizes enthalpy in thermodynamics is H. Enthalpy is defined by the equation H = U + pV, where U is internal energy, p is pressure, and V is volume.
The quantity was introduced to make energy accounting easier for systems operating at constant pressure, such as many chemical reactions carried out in open laboratory air. The pressure–volume term represents the energy associated with making room for a system against its surroundings. Enthalpy itself is a state function, so its change depends only on the initial and final states, not on the path between them.
A common mix-up is confusing H with heat. Heat is energy transferred because of a temperature difference, whereas enthalpy is a property of a system. Chemists often use enthalpy changes to describe reaction heat at constant pressure: an exothermic reaction has negative ΔH, while an endothermic reaction has positive ΔH. The symbol U refers to internal energy, G to Gibbs free energy, and F sometimes to Helmholtz free energy.