hydrothermal, a. [Hydro-, 1 + thermal.].
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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]
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of, pertaining to, or using the hot waters or steam found at special locations within the earth's crust; as, hydrothermal power generation. [PJC]