NOUN (3)

1. (economics) a market in which there are many buyers but only one seller;
- Example: "a monopoly on silver"
- Example: "when you have a monopoly you can ask any price you like"

2. exclusive control or possession of something;
- Example: "They have no monopoly on intelligence"

3. a board game in which players try to gain a monopoly on real estate as pieces advance around the board according to the throw of a die;

The Collaborative International Dictionary of English v.0.48:

Monopoly \Mo*nop"o*ly\, n.; pl. Monopolies. [L. monopolium,
   Gr. ?, ?; mo`nos alone + ? to sell.]
   1. The exclusive power, or privilege of selling a commodity;
      the exclusive power, right, or privilege of dealing in
      some article, or of trading in some market; sole command
      of the traffic in anything, however obtained; as, the
      proprietor of a patented article is given a monopoly of
      its sale for a limited time; chartered trading companies
      have sometimes had a monopoly of trade with remote
      regions; a combination of traders may get a monopoly of a
      particular product.
      [1913 Webster]

            Raleigh held a monopoly of cards, Essex a monopoly
            of sweet wines.                       --Macaulay.
      [1913 Webster]

   2. Exclusive possession; as, a monopoly of land.
      [1913 Webster]

            If I had a monopoly out, they would have part on 't.
                                                  --Shak.
      [1913 Webster]

   3. The commodity or other material thing to which the
      monopoly relates; as, tobacco is a monopoly in France.
      [Colloq.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

monopoly
    n 1: (economics) a market in which there are many buyers but
         only one seller; "a monopoly on silver"; "when you have a
         monopoly you can ask any price you like"
    2: exclusive control or possession of something; "They have no
       monopoly on intelligence"
    3: a board game in which players try to gain a monopoly on real
       estate as pieces advance around the board according to the
       throw of a die