The Collaborative International Dictionary of English v.0.48:

Isobar \I"so*bar\, n. [Iso- + Gr. ? weight.] (Phys. Geog.)
   A line connecting or marking places upon the surface of the
   earth where height of the barometer reduced to sea level is
   the same either at a given time, or for a certain period
   (mean height), as for a year; an isopiestic line. [Written
   also isobare.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

isobar
    n 1: (meteorology)an isogram connecting points having equal
         barometric pressure at a given time

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "isobar":
   air mass, anticyclone, artificial isotope, cold front, cold sector,
   cyclone, deuterium, front, heavy hydrogen, high,
   high-pressure area, isomer, isometric, isometric line,
   isopiestic line, isotherm, isothermal line, isotone, isotope, low,
   low-pressure area, nuclear isomer, occluded front, polar front,
   protium, radioactive isotope, radioisotope, squall line,
   stationary front, strontium 90, warm front, weather map,
   wind-shift line