A line drawn on a weather map or chart linking all points of equal or constant temperature.
A line on a map connecting points on the earth's surface having the same mean temperature or the same temperature at a given time.
A line representing changes of volume or pressure at constant temperature.
A line drawn on a weather map connecting points that have the same temperature. Each point can mark one temperature reading or an average of several readings.
Origin of isotherm
From the French adjective isotherme (“of equal temperature”), itself from Ancient Greek ἴσος (isos, “equal”) + θέρμη (thermē, “warmth”)