Substituent
Definition
An atom or group that replaces a hydrogen on a parent molecule: the branches and attachments that get named in organic nomenclature.
Technical definition
A substituent is an atom or functional group that replaces a hydrogen atom on a parent hydrocarbon chain or ring. Substituents are named as prefixes (methyl, chloro, hydroxy) in IUPAC nomenclature and strongly influence reactivity.
Example
In 2-chlorobutane, the chlorine atom is the substituent on carbon 2 of the butane parent chain.
Common confusion
Substituent and functional group overlap but are not identical. Substituent is a naming role (what is attached to the parent), while functional group is a reactivity role: the same -OH can be both.