"line connecting points on the earth having the same mean temperature," 1850, from French isotherme (von Humboldt, 1817), from Greek isos "equal, identical" (see iso-) + therme "heat" (see thermal (adj.)).
Entries linking to isotherm
iso-
before vowels often is-, word-forming element meaning "equal, similar, identical; isometric," from Greek isos "equal to, the same as; equally divided; fair, impartial (of persons); even, level (of ground)," as in isometor "like one's mother." In English used properly only with words of Greek origin; the Latin equivalent is equi- (see equi-).
thermal adj.
1756, "having to do with hot springs," from French thermal (Buffon), from Greek therme "heat, feverish heat," from PIE root *gwher- "to heat, warm." Sense of "having to do with heat" is first recorded 1837. The noun meaning "rising current of relatively warm air" is recorded from 1933.
isothermal adj.
1816, literally "of equal heat," from French isotherme (see isotherm) + -al (1). As a noun, "isothermal line," from 1849.