/ Dictionary / Index A ammonium: Meaning and Definition of Find definitions for: am•mo•ni•um Pronunciation: (u-mō'nē-um), [key] — n. Chem. the univalent ion, NH, or group, NH, which plays the part of a metal in the salt formed when ammonia reacts with an acid. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: ammonium (Thesaurus) ammonite ammonium acetate 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