Webster's English Dictionary

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ammonium

am.mo.nium \*-'mo--ne--*m, -ny*m\ n : an ion NH4+ or radical NH4 derived 
   from ammonia by combinati on with a hydrogen ion or atom and known in 
   compounds (as salts) that resemble in properties the compounds of the 
   alkali metals and in organic compounds (as quaternary ammonium compounds)