An isotopically labelled compound may be designated as isotopically deficient when the isotopic content of one or more elements has been depleted, i.e. when one or more nuclide(s) is (are) present in less than a natural ratio.
Sources:
Blue Book, p. 518 (https://www.acdlabs.com/iupac/nomenclature/)
PAC, 1981, 53, 1887. 'Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds' on page 1896 (https://doi.org/10.1351/pac198153101887)