substituent

Adjective, Noun

sub=stit=u=ent

CIDE DICTIONARY

substituent, n. [L. substituens, p. pr. See Substitute.].

Any atom, group, or radical substituted for another, or entering a molecule in place of some other part which is removed. [1913 Webster]

substituent, adj. & n. Chem.
--adj. (of a group of atoms) replacing another atom or group in a compound.
--n. such a group.

Etymology

L substituere substituent- (as SUBSTITUTE)