The Collaborative International Dictionary of English v.0.48:

Bimetallism \Bi*met"al*lism\, n. [F. bim['e]talisme.]
   The legalized use of two metals (as gold and silver) in the
   currency of a country, at a fixed relative value; -- in
   opposition to monometallism.
   [1913 Webster]

   Note: The words bim['e]tallisme and monom['e]tallisme are due
         to M. Cernuschi [1869]. --Littr['e].
         [1913 Webster]

WordNet (r) 3.0 (2006):

bimetallism
    n 1: a monetary standard under which the basic unit of currency
         is defined by stated amounts of two metals (usually gold
         and silver) with values set at a predetermined ratio