What means non equilibrium systems?
A system that is not in equilibrium.
It depends on the problem your dealing with, but can usually be figured out by the context. For example, a system not in mechanical equilibrium means that the sum of the forces does not equal zero. A system outside of thermal equilibrium means that there are temperature differences throughout. A semiconductor device is said to be out of equilibrium when there is a splitting of the quasi-Fermi levels. etc.
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