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