Describes a compound that has a macroscopic composition such that the isotopic ratio of nuclides for at least one element deviates measurably from that occurring in nature. It is either an isotopically substituted compound or an isotopically labelled compound.
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 1889 (https://doi.org/10.1351/pac198153101887)