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]