moiety

https://doi.org/10.1351/goldbook.M03968

In physical organic chemistry moiety is generally used to signify part of a molecule, e.g. in an ester R¹COOR² the alcohol moiety is R²O. The term should not be used for a small fragment of a molecule.

Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1141 (https://doi.org/10.1351/pac199466051077)