preferential voting

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A system of voting in which the voter ranks candidates in order of preference.

from the GNU version of the Collaborative International Dictionary of English.

• (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.