Of the same degree of heat; of the same temperature; in physical geography, pertaining to or marking equality of temperature; exhibiting the geographical distribution of temperature: as, an isothermal line; the isothermal relations of different continents; an isothermal chart. Also isothermous.n. An isothermal line; an isotherm.In crystallography, applied to the lines on a crystal face, or more generally to the surfaces in a crystal, which measure the rate of heat-conductivity, these lines or surfaces being characterized by equal degrees of temperature when heat is applied at a certain point.n. In thermodynamics, a line or curve determined by the equation of state (pv = RT) of a system, when T has any constant value; a line of equal temperatures.n. In spectrophotometry, a curve showing the distribution of intensities in the spectrum of a source of light maintained at constant temperature. Also called isotherm.