A molecular entity that differs only in isotopic composition (number of isotopic substitutions), e.g.
CH4,
CH3D,
CH2D2.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1132 (https://doi.org/10.1351/pac199466051077)