WordNet (r) 3.0 (2006):

runoff
    n 1: the occurrence of surplus liquid (as water) exceeding the
         limit or capacity [syn: overflow, runoff, overspill]
    2: a final election to resolve an earlier election that did not
       produce a winner

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "runoff":
   bout, by-election, caucus, closed primary, congressional election,
   contest, contested election, decathlon, defluxion, direct primary,
   discharge, double-header, doubles, drain, drainage, effluence,
   efflux, effluxion, effusion, election, event, exhalation, exhaust,
   flowoff, foursome, game, games of chance, general election, go,
   gush, mandatory primary, match, meet, nonpartisan primary,
   open primary, optional primary, outfall, outflow, outflowing,
   outpour, outpouring, partisan election, pentathlon, play, play-off,
   preference primary, presidential election,
   presidential preference primary, presidential primary, primary,
   primary election, race, referendum, runoff election,
   runoff primary, singles, sport, threesome, twosome