Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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