The Collaborative International Dictionary
Geothermometer
Geothermometer \Ge`o*ther*mom"e*ter\, n. [Gr. ge`a, gh^, the earth + E. thermometer.] (Physics) A thermometer specially constructed for measuring temperetures at a depth below the surface of the ground.
Wiktionary
geothermometer
n. A thermometer designed for measuring temperatures below ground level.