The Collaborative International Dictionary of English v.0.48:

Isothermal \I`so*ther"mal\, a. [Cf. F. isotherme.]
      (a) Relating to equality of temperature, or an isotherm.
      (b) (Phys. Geog.) Having reference to the geographical
          distribution of temperature, as exhibited by means of
          isotherms; as, an isothermal line; an isothermal
          chart.
          [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

isothermal
    adj 1: of a process or change taking place at constant
           temperature