In thermodynamics, equilibrium is maintained between the components of a system or between multiple systems when the net exchange of mechanical, chemical and thermal energy between the components is zero. Systems in equilibrium have no net macroscopic flow of any form of matter or energy.
An isolated system is one that has no long-range interactions with its surroundings. It is completely shielded from all exchanges of mass and energy with the outside world. Such an isolated thermodynamic system is said to be in equilibrium when it is isothermal and isobaric. All points in the system are at the same temperature and pressure. The system is also spatially homogeneous in all other regards; there are no concentration gradients between the different constituents that make up the system.
Multiple systems that are in equilibrium are extensions of single systems; they are also devoid of any net exchange of physical properties from one system to the other. The definition of such simple systems in equilibrium is the zeroth law of thermodynamics.
Systems that are not in equilibrium tend toward equilibrium. For example, a rod of metal that is hot on one end and cold on the other can be viewed as a system that is not in thermal equilibrium. If the rod is completely insulated against any loss of energy to the environment, the heat redistributes itself until the entire rod is at the same temperature.