1816, literally "of equal heat," from French isotherme (see isotherm) + -al (1). As a noun, "isothermal line," from 1849.
Entries linking to isothermal
isotherm n.
"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.)).
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).