from
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]