An azeotropic mixture, also known as a constant-boiling-point mixture, is a solution of two or more liquids which, upon distillation, behaves as a pure liquid: its boiling point is invariable at a given pressure, and the composition of the vapor phase is the same as that of the liquid. This means that the liquids cannot be separated by distillation.

Two types of azeotropes are known: minimum-boiling and maximum-boiling, the former being more common. One of the best known minimum-boiling azeotropse is the ethanol-water system which at a pressure of 1 atmosphere occurs at 89.4 mole percent ethanol and 78.2°C.