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Equilibrium may refer to a variety of concepts, including thermodynamic equilibrium and chemical equilibrium.

Thermodynamic equilibrium is a concept within thermodynamics. Thermodynamic equilibrium is a state where there is no net flow of matter or energy within a system.

Chemical equilibrium is discussed in the context of chemical reactions. At chemical equilibrium, the concentrations of all chemical species (reactants and products) stays constant. Note that chemical equilibrium does not necessarily mean that no reactions are happening - rather, the reaction rates of the "forward" and "backward" reactions are equal, resulting in no net change in concentrations.

See also

The Wikipedia page on thermodynamic equilibrium

The Wikipedia page on chemical equilibrium