The Collaborative International Dictionary of English v.0.48:

Hydrothermal \Hy`dro*ther"mal\, a. [Hydro-, 1 + thermal.]
   1. Of or pertaining to hot water; -- used esp. with reference
      to the action of heated waters in dissolving,
      redepositing, and otherwise producing mineral changes
      within the crust of the globe.
      [1913 Webster]

   2. of, pertaining to, or using the hot waters or steam found
      at special locations within the earth's crust; as,
      hydrothermal power generation.
      [PJC]