WordNet (r) 3.0 (2006):

proportional representation
    n 1: representation of all parties in proportion to their
         popular vote

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "proportional representation":
   Australian ballot, Hare system, PR, absentee voting, aye, ballot,
   ballot-box stuffing, canvass, canvassing, card voting,
   casting vote, colonization, counting heads, cumulative system,
   cumulative voting, deciding vote, division, election fraud,
   enfranchisement, fagot vote, floating, franchise, graveyard vote,
   hand vote, list system, nay, no, nontransferable vote, plebiscite,
   plebiscitum, plumper, plural system, plural vote, poll, polling,
   preferential system, preferential voting, proxy, proxy voting,
   record vote, referendum, repeating, representation, right to vote,
   rising vote, say, secret ballot, show of hands, single system,
   single transferrable vote, single vote, single-member district,
   snap vote, straw vote, suffrage, transferable vote, viva voce,
   voice, voice vote, vote, voting, voting machine, voting right,
   write-in, write-in vote, yea, yeas and nays, yes