Source : Webster's Revised Unabridged Dictionary (1913)

Isotherm \I"so*therm\, n. [Iso- + Gr. ? heat, fr. ? hot.] (Phys.
   Geog.)
   A line connecting or marking points on the earth's surface
   having the same temperature. This may be the temperature for
   a given time of observation, or the mean temperature for a
   year or other period. Also, a similar line based on the
   distribution of temperature in the ocean.

Source : WordNet®

isotherm
     n : an isogram connecting points having the same temperature at
         a given time