In thermodynamics and molecular chemistry, the enthalpy or heat content (denoted as H, h, or rarely as χ) is a quotient or description of thermodynamic potential of a system, which can be used to calculate the "useful" work obtainable from a closed thermodynamic system under constant pressure and entropy. The term enthalpy was composed of the prefix en-, meaning to "put into" and the Greek word -thalpein, meaning "to heat", although the original definition is thought to have stemmed fro… (More on Enthalpy)