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Thermodynamic equilibrium is an axiomatic concept of classical thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by permeable walls. In thermodynamic equilibrium there are no net macroscopic flows of matter or of energy, either within a system or between systems. In a system in its own state of internal thermodynamic equilibrium, no macroscopic change occurs. Systems in mutual thermodynamic equilibrium are simultaneously mutually in thermal, mechanical, chemical, and radiative equilibria. Systems can be in one kind of mutual equilibrium, though not in others. In thermodynamic equilibrium, all kinds of equilibrium hold at once and indefinitely, until disturbed by a thermodynamic operation. In a macroscopic equilibrium, almost or perfectly, exactly balanced microscopic exchanges occur; this is part of the notion of macroscopic equilibrium. An isolated thermodynamic system in its own state of internal thermodynamic equilibrium has a uniform temperature. If its surroundings impose some unchanging long range force field on it, it may consist of one phase or may exhibit several spatially unchanging internal phases. If its surroundings impose no long range force field on it, then either (1) it is spatially homogeneous, with all intensive properties being uniform; or (2) it has several internal phases, which may exhibit indefinitely persistent continuous spontaneous microscopic or mesoscopic fluctuations. In non-equilibrium systems, by contrast, there are net flows of matter or energy. If such changes can be triggered to occur in a system in which they are not already occurring, it is said to be in a metastable equilibrium. It is an axiom of thermodynamics that when a body of material starts from a non-equilibrium state of non-homogeneity or chemical non-equilibrium, and, by a thermodynamic operation, is then isolated, it spontaneously evolves towards its own internal state of thermodynamic equilibrium. This axiom is presupposed by the second law of thermodynamics, which restricts what can happen when a system, having reached thermodynamic equilibrium, with a well defined entropy, is subject to a thermodynamic operation.