A species of atom, characterized by its @M03726@, @A00499@ and nuclear energy state, provided that the @M03780@ in that state is long enough to be observable.
Orange Book, 2nd ed., p. 233 [Terms] [Book]
PAC, 1982, 54, 1533. (Glossary of terms used in nuclear analytical chemistry (Provisional)) on page 1547 [Terms] [Paper]
PAC, 1994, 66, 2513. (Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)) on page 2522 [Terms] [Paper]
Red Book, 3rd ed., p. 35 [Terms] [Book]