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.
Orange Book, 2nd ed., p. 233 (http://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)