- noun one or more atomic nuclei of the same element, defined by having the same number of protons and neutrons and the same energy content
- Any species of atom that exits for an interval greater than a given minimum, such as one yoctosecond. Nuclides are distinguished by their atomic mass, atomic number, and energy state of the nucleus. For example, carbon-14 is a nuclide of carbon. A nuclide which has the same atomic mass as another, but whose atomic number is different is called an isobar (2).