third party

third party
    n 1: someone other than the principals who are involved in a
         transaction
    2: a political party organized in opposition to the major
       parties in a two-party system
    

52 Moby Thesaurus words for "third party":
      American Party, Anti-Monopoly Party, Bull Moose Party,
      Communist Party, Conservative Party, Constitutional Union Party,
      Democratic Party, Democratic-Republican Party, Farmer-Labor Party,
      Federalist Party, Free Soil Party, Greenback Party,
      Know-Nothing Party, Labour Party, Liberal Party,
      Liberal Republican Party, Liberty Party, National Republican Party,
      Populist Party, Progressive Party, Prohibition Party,
      Republican Party, Socialist Labor Party, Socialist Party,
      Socialist Workers Party, Tammany Hall, Tory Party, Whig Party,
      arbiter, arbitrator, camp, city hall, faction,
      impartial arbitrator, judge, machine, magistrate, major party,
      minor party, moderator, multiple party system, one-party system,
      opposition party, party, party in power, political machine,
      political party, referee, splinter party, two-party system, umpire,
      unbiased observer