The Collaborative International Dictionary of English v.0.48:

Isotherm \I"so*therm\, n. [Iso- + Gr. ? heat, fr. ? hot.]
   1. (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.
      [1913 Webster]

   2. (Physics) A line on a graph connecting points representing
      states having the same temperature; an isothermal line.
      [PJC]

WordNet (r) 3.0 (2006):

isotherm
    n 1: (meteorology) an isogram connecting points having the same
         temperature at a given time

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "isotherm":
   air mass, anticyclone, cold front, cold sector, cyclone, front,
   high, high-pressure area, isobar, isometric, isometric line,
   isopiestic line, isothermal line, low, low-pressure area,
   occluded front, polar front, squall line, stationary front,
   warm front, weather map, wind-shift line