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):

water of hydration
    n 1: the water present in hydrated compounds [syn: water of
         crystallization, water of crystallisation, water of
         hydration]