An isobar can refer to two different concepts: one in nuclear physics and one in meteorology.

In nuclear physics, an isobar is a term used to describe atoms (nuclides) of different chemical elements that have the same number of nucleons. Isobars differ in atomic number (or number of protons) but have the same mass number. For example, 40S, 40Cl, 40Ar, 40K, and 40Ca are a series of isobars, all containing 40 nucleons but varying numbers of protons and neutrons).

In meteorology, an isobar is a line on a weather map or chart that connects points of equal atmospheric pressure reduced to sea level. Isobars help meteorologists locate frontal boundaries because fronts are located in troughs of low pressure.

To summarize, an isobar can refer to:

• In nuclear physics: atoms of different chemical elements that have the same number of nucleons but different atomic numbers.
• In meteorology: a line on a weather map or chart that connects points of equal atmospheric pressure reduced to sea level.