line of equal or constant pressure on a graph, plot, or map

In meteorology, an isobar is a line on a map that connects areas with the same atmospheric pressure. Wind is generally perpendicular to isobars. When looking at the difference in pressure, it is also possible to estimate wind direction and speed. The first person to use isobars was Alexander Buchan (1829-1907).

Isobar lines, showing a hurricane over the British Isles.