A specifically labelled compound in which the isotopically substituted molecule has only one isotopically modified atom e.g.
CH3–CH[2H]–OH .
Sources:
Blue Book, p. 515 (http://www.acdlabs.com/iupac/nomenclature/)
PAC, 1981, 53, 1887. 'Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds' on page 1892 (https://doi.org/10.1351/pac198153101887)