From Glossary of Meteorology
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A line of equal or constant pressure; an isopleth of pressure.

In meteorology, it most often refers to a line drawn through all points of equal atmospheric pressure along a given reference surface, such as a constant-height surface (notably mean sea level on surface charts), an isentropic surface, the vertical plane of a synoptic cross section, etc. The pattern of isobars has always been a main feature of surface-chart analysis. Isobars are usually drawn at intervals of one millibar or more, depending on the scale needed to identify or illustrate a specific meteorological pattern.