What is meant when a system has reached equilibrium?

In chemistry, the term equilibrium refers to a chemical reaction occurring in which there is no net change in the concentration of reactants and products. The quantities of products and reactants achieve a constant ratio.

The term chemical equilibrium applies to reactions occurring in gaseous or aqueous states. When reactants are solid or liquid, the concentrations do not change. At equilibrium, the reactions continue to occur, but they occur in both a forward and reverse direction at a constant state. The products are consumed to create reactants at the same rate they are produced. Physical changes, such as increasing the temperature, affect equilibrium.