Isobars

A line drawn on a weather map connecting points of equal pressure is called an isobar. That means, that at every point along a given isobar, the values of pressure are the same. The isobars are generated from mean sea level pressure reports and the pressure values are given in millibars.

The above is a map of isobars indicating areas of equal pressure across parts of Europe.
http://www.windfinder.com/grafiken/isobars/isobars_central_europe_1.gif