The Collaborative International Dictionary
Water of hydration
Hydration \Hy*dra"tion\, n. (Chem.) The act of becoming, or state of being, a hydrate.
Water of hydration (Chem.), water chemically combined with some substance to form a hydrate; -- distinguished from water of crystallization.
WordNet
water of hydration
n. the water present in hydrated compounds [syn: water of crystallization, water of crystallisation]