An azeotrope is a mixture of two or more compounds (molecules) which at a specific composition has a vapor in equilibrium with that liquid with the same composition as the liquid. This means that you cannot change the composition of that liquid at its azeotropic composition by simple boiling. This constant composition will (almost always) occur at a minimum or maximum temperature over the composition space. The exception for binary mixtures is the rare double azeotrope. The exception for three or more components is the intermediate boiling azeotrope (saddle azeotrope).