The Collaborative International Dictionary of English v.0.48:
Hydration \Hy*dra"tion\, n. (Chem.) The act of becoming, or state of being, a hydrate. [1913 Webster] Water of hydration (Chem.), water chemically combined with some substance to form a hydrate; -- distinguished from water of crystallization. [1913 Webster]WordNet (r) 3.0 (2006):
hydration n 1: the process of combining with water; usually reversible