Exergonic (from the suffix exo-, derived for the Greek word ἔξω exō, "outside" and the suffix -ergonic, derived from the Greek word ἔργον ergon, "work") means "releasing energy in the form of work". In thermodynamics, work is defined as the energy moving from the system (the internal region) to the surroundings (the external region) during a given process. An exergonic process is one in which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process.

Constant pressure, constant temperature reactions are exergonic iff the Gibbs free energy is negative (∆G < 0).

All physical and chemical systems in the universe follow the second law of thermodynamics and proceed in a downhill, i.e., exergonic, direction. Thus, left to itself, any physical or chemical system will proceed, according to the second law of thermodynamics, in a direction that tends to lower the free energy of the system, and thus to expend energy in the form of work. These reactions occur spontaneously.