The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ammonium
    n 1: the ion NH4 derived from ammonia; behaves in many respects
         like an alkali metal ion [syn: ammonium, ammonium ion]