/ Dictionary / Index S sulfonium: Meaning and Definition of Find definitions for: sul•fo•ni•um Pronunciation: (sul-fō'nē-um), [key] — n. Chem. the positively charged group HS⫀, its salts, or their substitute products. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. sulfonic acid sulfonyl Related Content Daily Word Quiz: labyrinth Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language