characteristic group

in organic nomenclature
A single heteroatom, for example, –Cl and =O; a heteroatom bearing one or more hydrogen atoms or other heteroatoms, for example, –NH2, –OH, –SO3H, –PO3H2 and IO2 ; or a heteroatomic group attached to or containing a single carbon atom, for example, –CHO, –C≡N, –COOH and –NCO , attached to a parent hydride.
Blue Book (Guide), p. 13