ammonium Ammonitoidea • ammonium alum · · · Info · Definitions · Synonyms · Characters · · ammonium · · Text Latin · · Script 8 · · Length 4 · · Syllables Definitions · · · (noun) the ion NH4 derived from ammonia; behaves in many respects like an alkali metal ion. Synonyms: ammonium ion. · · Noun Definition 1 Synonyms · · · ammonium ion · · Synonym 1 Characters · · a latin small letter a U+0061 i latin small letter i U+0069 m latin small letter m U+006D n latin small letter n U+006E o latin small letter o U+006F u latin small letter u U+0075