A thermodynamic system that is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium.
A thermodynamic system is in equilibrium when there are:
(1) Thermal equilibrium : when two systems in thermal contact with each other do not exchange energy by heat since their temperatures are the same.
(2) Mechanical equilibrium : when pressures of the two systems are the same.
(3) Chemical equilibrium : when chemical potentials of the two systems are the same.