As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another. That is to say, anhydrous matter is prepared or used as a form with no water, and specifically the form of a chemical without water of crystallization.

Les anhydres sont des composés qui ne contiennent pas d'eau libre mais gardant une fraction d'eau de constitution, contrairement aux hydrates.