The Collaborative International Dictionary of English v.0.48:

Republican \Re*pub"lic*an\ (-l?-kan), a. [F. r['e]publicain.]
   1. Of or pertaining to a republic.
      [1913 Webster]

            The Roman emperors were republican magistrates named
            by the senate.                        --Macaulay.
      [1913 Webster]

   2. Consonant with the principles of a republic; as,
      republican sentiments or opinions; republican manners.
      [1913 Webster]

   Republican party. (U.S. Politics)
      (a) An earlier name of the Democratic party when it was
          opposed to the Federal party. Thomas Jefferson was its
          great leader.
      (b) One of the existing great parties. It was organized in
          1856 by a combination of voters from other parties for
          the purpose of opposing the extension of slavery, and
          in 1860 it elected Abraham Lincoln president.
          [1913 Webster]

WordNet (r) 3.0 (2006):

Republican Party
    n 1: the younger of two major political parties in the United
         States; GOP is an acronym for grand old party [syn:
         Republican Party, GOP]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "Republican 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,
   Socialist Labor Party, Socialist Party, Socialist Workers Party,
   Tammany Hall, Tory Party, Whig Party, camp, city hall, faction,
   machine, major party, minor party, multiple party system,
   one-party system, opposition party, party, party in power,
   political machine, political party, splinter party, third party,
   two-party system