A species of atom, characterized by its mass number, atomic number and nuclear energy state, provided that the mean life in that state is long enough to be observable.
Sources:
Orange Book, 2nd ed., p. 233 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1547 (https://doi.org/10.1351/pac198254081533)
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2522 (https://doi.org/10.1351/pac199466122513)
Red Book, 3rd ed., p. 35 (https://doi.org/10.1515/pac-2014-0718)