Isothermal \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.

Source: Webster's Revised Unabridged Dictionary (1913)