Main Entry: am·mo·ni·um Pronunciation: &-'mO-nE-&m Function: noun : an ion NH4+derived from ammonia bycombination with a hydrogen ion and known in organic compounds (as quaternary ammonium compounds) and in compounds (as salts) that resemble in properties the compounds of the alkali metals
ammonium am·mo·ni·um (ə-mō'nē-əm) n. The univalent radical NH4+, that is derived from ammonia and that reacts as a univalent metal in forming ammonium compounds.