http://en.wikipedia.org/wiki/Instant_runoff_voting :
Instant-runoff voting (IRV), also known as preferential voting, the alternative vote and ranked choice voting, is a voting system used to elect one winner. Voters rank candidates in order of preference, and their ballots are counted as one vote for their first choice candidate. If a candidate secures a majority of votes cast, that candidate wins. Otherwise, the candidate with the fewest votes is eliminated. A new round of counting takes place, with each ballot counted as one vote for the advancing candidate who is ranked highest on that ballot. This process continues until the winning candidate receives a majority of the vote against the remaining candidates.