The Collaborative International Dictionary of English v.0.48:

Hydro \Hy"dro\, n.
   1. A hydro-a["e]roplane. [obsolescent]
      [Webster 1913 Suppl.]

   2. hydroelectric power; -- also used attributively.
      [PJC] Hydro

The Collaborative International Dictionary of English v.0.48:

Hydro- \Hy"dro-\, Hydr- \Hy"dr-\
   1. A combining form from Gr. ?, ?, water (see Hydra).
      [1913 Webster]

   2. (Chem.) A combining form of hydrogen, indicating hydrogen
      as an ingredient, as hydrochloric; or a reduction product
      obtained by hydrogen, as hydroquinone.
      [1913 Webster]