An isobar refers to all of the nuclides of all elements that have a particular atomic mass. For example, helium-6 is one of the many isotopes of helium. The isobars of helium-6 are hydrogen-6, lithium-6, beryllium-6, and boron-6. (Note, hydrogen-6, beryllium-6, and boron-6 are unbelievably short-lived).