noun
1. : the negative ion (as the sulfate ion SO 4 -- ) of an acid : anion — now used especially by analysts
2. : a radical formed by removal of all hydroxyl or analogous groups (as mercapto) from an acid
benzoyl C 6 H 5 CO — is the acid radical corresponding to benzoic acid C 6 H 5 COOH
sulfuryl >SO 2 is the acid radical of sulfuric acid H 2 SO 4
— compare acyl