NOUN (1)

1. the process of combining with water; usually reversible;

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]