ammonium

from 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}]

from 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]