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.
Lines on a map connecting points of equal atmospheric pressure at a particular time.
The lines on a plot (map) showing continuous level. In a speaker plot, it shows the pattern where the sound is a given level lower than the direct center (on-axis) at a given frequency. For microphones, the isobars on the plot show the relative sensitivity at different angles off axis (based on frequency). Usually isobars are given in increments of 3 or 10dB.