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The Collaborative International Dictionary
Isobathytherm

Isobathytherm \I`so*bath"y*therm\, n. [Iso- + Gr. ? deep + ? heat.] (Phys. Geog.) A line connecting the points on the surface of the earth where a certain temperature is found at the same depth.

Wiktionary
isobathytherm

n. A line connecting the points on the Earth's surface where a certain temperature is found at the same depth.