Isotherm

n. [Gr. ἴσος, equal, and θέρμη, heat.] (Phys. Geog.) An imaginary line over the earth’s surface passing through points having the same mean annual temperature, so that a series of such lines exhibits the distribution of temperature over the earth’s surface; in a less restricted sense, any similar line passing through points having the same mean temperature, but not exclusively the annual mean temperature; also, a similar line based on the distribution of temperature in the waters of the ocean. A. K. Johnston.