An isobar is a curve drawn through points of equal pressure. For example, it could be a line joining states of equal pressure in a graph representing thermodynamic processes. The term is commonly used in meteorology where an isobar is a curve joining points of equal atmospheric pressure on a given reference surface, e.g., sea level.
Note that isobar is also used in nuclear physics to describe nuclides having the same number of nucleons, but with different numbers of protons and neutrons.