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 ]