Source : Webster's Revised Unabridged Dictionary (1913)

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}.]

Isobar \I"so*bar\, n.
   The quality or state of being equal in weight, especially in
   atmospheric pressure. Also, the theory, method, or
   application of isobaric science.

Source : WordNet®

isobar
     n : an isogram connecting points having equal barometric
         pressure at a given time