Source : Webster's Revised Unabridged Dictionary (1913)

Hydro- \Hy"dro-\, Hydr- \Hy"dr-\
   1. A combining form from Gr. ?, ?, water (see {Hydra}).

   2. (Chem.) A combining form of hydrogen, indicating hydrogen
      as an ingredient, as hydrochloric; or a reduction product
      obtained by hydrogen, as hydroquinone.