substituent atom (group)

https://doi.org/10.1351/goldbook.S06076
An atom (group) that replaces one or more hydrogen atoms attached to a parent structure or characteristic group except for hydrogen atoms attached to a chalcogen atom.
Sources:
Blue Book Guide, p. 13 [Terms] [Book]
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1169 [Terms] [Paper]