Source : Webster's Revised Unabridged Dictionary (1913)

Preferential voting \Preferential voting\ (Political Science)
   A system of voting, as at primaries, in which the voters are
   allowed to indicate on their ballots their preference
   (usually their first and second choices) between two or more
   candidates for an office, so that if no candidate receives a
   majority of first choices the one receiving the greatest
   number of first and second choices together in nominated or
   elected.