A solution of two or more liquids, the composition of which does not change upon distillation. The composition of the liquid phase at the boiling point is identical to that of the vapor in equilibrium with it, and such mixtures or azeotropes form constant-boiling solutions. The exact composition of the azeotrope changes if the boiling point is altered by a change in the external pressure.

A solution of two components which form an azeotrope may be separated by distillation into one pure component and the azeotrope, but not into two pure components. Standard solutions are often prepared by distillation of aqueous solutions until the azeotropic composition is reached. At 1 atm (760 mmHg or approximately 100 kilopascals) pressure, hydrogen chloride and water form an azeotrope containing 20.24% by weight of HCl.