Source : Webster's Revised Unabridged Dictionary (1913)

Hydrothermal \Hy`dro*ther"mal\, a. [Hydro-, 1 + thermal.]
   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.