Functional group
The substituent or moiety of a molecule, appeared as a molecule or a group of molecule, can causes the distinctive characteristic chemical reaction.
The functional group will shared the same or similar chemical properties regardless of the rest of the molecule's composition. However, the reactivity of a functional group can be modified by other nearby functional group.
Hydrocarbyl
Supprisingly, the hydrocarbon as a branch of a hydrocarbon chain can actually consider a functional group, defined as hydrocarbyl.