In thermodynamics, enthalpy, symbolized by H, summarized as ‘heat content’, is defined as the sum of the internal energy of a thermodynamic system plus the energy associated with work done by the system on the atmosphere, i.e. the product of the pressure times the volume. Enthalpy reflects the number and kinds of chemical bonds in the reactants and products. [1] In equation form, enthalpy is defined as follows:

$H = U + PV \,$

The quantity H, equal to the internal energy plus the pressure volume energy, was first recognized by American engineer Willard Gibbs as playing an important role for processes occurring at constant pressure, and was called by him ‘heat content’.

Etymology
Italian physical chemist Salvatore Califano states that the term was introduced by Dutch physicist Heike Kamerlingh-Onnes at the first meeting of the Institute of Refrigeration in Paris in 1908. [5]

Alternatively, Canadian physical chemist Keith Laidler stated that Onnes introduced the term "enthalpy" in 1909, symbol H, from the Greek εν (en) ‘in’ and θαλπος (thalpos) ‘to heat’, which combined define the word enthalpos, to warm within. [4]

American protein thermodynamicist Donald Haynie, defines enthalpy, in the context of biological thermodynamics (chnops-thermodynamics), as a ‘thermodynamic state function usually measured as heat transferred to or from a system at constant pressure.’ [2] With the constraints of constant pressure and amount of substance, the differential change in enthalphy dH, of a system evolving in such conditions, equals the amount of heat dQP exchanged with the surroundings.’ [3]

Enthalpy-entropy compensation

References
1. Lehninger, A.L., Nelson, D.L., & Cox, M.M. (1993). Principles of Biochemistry, 2nd ed. New York: Worth Publishers.
2. Haynie, Donald T. (2001). Biological Thermodynamics. Cambridge: Cambridge University Press.
3. Perrot, P. (1998). A to Z of Thermodynamics. Oxford: Oxford University Press.
4. Laidler, Keith J. (1993). The World of Physical Chemistry (pg. 110). Oxford University Press.
5. Califano, Salvatore. (2012). Pathways to Modern Chemical Physics (pg. 7). Springer.