geotherm
/ˈdʒiːə(ʊ)θəːm/Geology
noun
Originally: = isogeotherm. Now usually: a graph of temperature as a function of depth in the earth's crust; a geothermal gradient.
Origin
Early 20th century; earliest use found in John Joly (1857–1933), geologist and physicist. From geo- + -therm; perhaps originally short for isogeotherm.