Describes a compound that has a composition such that essentially all the molecules of the compound have only the indicated nuclide(s) at each designated position. For all other positions, the absence of nuclide(s) indication means that the nuclide(s) composition is the natural one.
Sources:
Blue Book, p. 514 (https://www.acdlabs.com/iupac/nomenclature/)
PAC, 1981, 53, 1887. 'Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds' on page 1890 (https://doi.org/10.1351/pac198153101887)