Equilibria

TL;DR: Equilibrium is a state where chemical processes stop, with reactants, products and intermediates present in defined concentrations determined by the equilibrium constant.

Abstract: The concept that chemical processes may not evolve "to completion" and instead tend to stop midway results in an "equilibrium" where reactants, products and potentially intermediates might be present. Identifying the concentrations of these populations requires rigorous definition of new reaction parameters (e.g. the equilibrium constant).