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