Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

isothermal
     adj : of a process or change taking place at constant temperature