A hydrate is a compound (usually ionic) containing a definite proportion of water, known as water of crystallization, which may be bound as a ligand to the cation (positive ion), to the anion (negative ion), or both. Other hydrates, with more or less water, may be formed under different conditions. Water may be lost from a hydrate spontaneously (efflorescence) or by heating, the compound becoming anhydrous.

Common hydrates include copper (II) sulfate, CuSO₄.5H₂O; sodium carbonate, Na₂CO₃.10H₂O, and the alums.