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.
Source:
PAC, 1994, 66, 2513
(Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994))
on page 2522
Transformed and rewritten from PDF version (entry
http://www.iupac.org/goldbook/N04257.pdf)
by: Miloslav Nic, Jiri Jirat, Bedrich Kosata, ICT Prague, Czech Republic