Sulphonic \Sul*phon"ic\, a. (Chem.) Pertaining to, or derived from, a sulphone; -- used specifically to designate any one of a series of acids (regarded as acid ethereal salts of sulphurous acid) obtained by the oxidation of the mercaptans, or by treating sulphuric acid with certain aromatic bases (as benzene); as, phenyl sulphonic acid, C6H5.SO2.OH, a stable colorless crystalline substance.

Sulphonic group (Chem.), the hypothetical radical, SO2.OH, the characteristic residue of sulphonic acids.

Source: Webster's Revised Unabridged Dictionary (1913)