/ Dictionary / Index O -onium: Meaning and Definition of Find definitions for: -onium Pronunciation: [key] — Chem. Chem. a suffix used in the names of complex cations: oxonium. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Onitsha Onkelos Related Content Daily Word Quiz: gregarious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language