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.
PAC, 1994, 66, 2513 (Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)) on page 2522
Red Book, p. 35
Orange Book, p. 233
PAC, 1982, 54, 1533 (Glossary of terms used in nuclear analytical chemistry (Provisional)) on page 1547