Isobars are atoms of different chemical elements that have the same number of nucleons, meaning they have the same mass number but differ in atomic 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). The term "isobars" comes from the Greek word isos, meaning "equal," and baros, meaning "weight").

Isobars can also refer to lines of equal atmospheric pressure drawn on a meteorological map. Each line passes through a pressure of a given value, and the distance between isobars indicates the barometric gradient, which is the degree of pressure change over a given distance. Isobars are used in weather forecasting to identify areas of high and low pressure and to predict the movement of weather systems.

In summary, isobars can refer to two different things:

• Atoms of different chemical elements that have the same number of nucleons, meaning they have the same mass number but differ in atomic number).
• Lines of equal atmospheric pressure drawn on a meteorological map, used in weather forecasting to identify areas of high and low pressure and to predict the movement of weather systems.