The Collaborative International Dictionary of English v.0.48:

Hydrotherapy \Hy`dro*ther"a*py\, n. [Hydro-, 1 + therapy.]
   (Med.)
   See Hydropathy.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hydrotherapy
    n 1: the internal and external use of water in the treatment of
         disease [syn: hydropathy, hydrotherapy]