The Collaborative International Dictionary of English v.0.48:

Hydrous \Hy"drous\, a. [Gr. "y`dwr water.]
   1. Containing water; watery.
      [1913 Webster]

   2. (Chem.) Containing water of hydration or crystallization.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hydrous
    adj 1: containing combined water (especially water of
           crystallization as in a hydrate) [syn: hydrous,
           hydrated] [ant: anhydrous]