Source : Webster's Revised Unabridged Dictionary (1913)

Ammonium \Am*mo"ni*um\, n. [See {Ammonia}.] (Chem.)
   A compound radical, {NH4}, having the chemical relations of a
   strongly basic element like the alkali metals.

Source : WordNet®

ammonium
     n : the ion NH4 derived from ammonia; behaves in many respects
         like an alkali metal ion [syn: {ammonium ion}]