Azeotrope

n. liquid mixture with a fixed boiling point that does not decompose during distillation (Chemistry)

An azeotrope is a mixture of two or more pure compounds (chemicals) in such a ratio that its composition cannot be changed by simple distillation. This is because when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture of liquids. Because composition is unchanged by boiling, azeotropes are also known as constant boiling mixtures (especially in older texts). The word azeotrope comes from the Greek words "α ζειν τρόπος", meaning "no change on boiling".
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adj. azeotrope, liquid mixture with a fixed boiling point that does not decompose during distillation (Chemistry)

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A solution that does not change composition when distilled. For example, if a 95% (w/w) ethanol solution in water is boilled, the vapor produced also is 95% ethanol- and it is not possible to obtain higher percentages of ethanol by distillation.

AZEOTROPO