\I`so*ther"mal\, a. [Cf. F. isotherme.]
(a) Relating to equality of temperature.
(b) (Phys. Geog.) Having reference to the geographical
    distribution of temperature, as exhibited by means of
    isotherms; as, an isothermal line; an isothermal chart.
{Isothermal line}.
(a) An isotherm.
(b) A line drawn on a diagram of energy such that its
    ordinates represent the pressures of a substance
    corresponding to various volumes, while the absolute
    temperature is maintained at a constant value.
{Isothermal zones}, spaces on opposite sides of the equator
   having the same mean temperature, and bounded by
   corresponding isothermal lines.