Instant-runoff voting

Instant-runoff voting (IRV), also known as the alternative vote (AV) or transferable vote, is a voting system used in single-seat elections when there are more than two candidates. (It is also sometimes referred to as "ranked-choice voting" (RCV) or "preferential voting", though there are many other preferential voting systems, all of which use ranked-choice ballots.)

Instead of voting only for a single candidate, in IRV, voters can rank the candidates in order of preference. Ballots are initially counted for each elector's top choice. If a candidate secures more than half of these votes, that candidate wins. Otherwise, the candidate in last place is eliminated and removed from consideration. The top remaining choices on all the ballots are then counted again. This process repeats until one candidate is the top remaining choice of a majority of the voters. When the field is reduced to two, it has become an "instant runoff" that allows a comparison of the top two candidates head-to-head.

IRV has the effect of avoiding split votes when multiple candidates earn support from like-minded voters. For example, suppose there are two similar candidates A & B, and a third opposing candidate C, with first-preference vote totals of 35% for candidate A, 25% for B and 40% for C. In a plurality voting system, candidate C may win with 40% of the votes, even though 60% of electors prefer both A and B over C. Alternatively, voters are pressured to choose the seemingly stronger candidate of either A or B, despite personal preference for the other, in order to help ensure the defeat of C. With IRV, the electors backing B as their first choice can rank A second, which means candidate A will win by 60% to 40% over C despite the split vote in first choices.

Instant-runoff voting is used in national elections in several countries. For example, it is used to elect members of the Australian House of Representatives and most Australian state legislatures, the President of India, members of legislative councils in India, the President of Ireland, and the parliament in Papua New Guinea. The system is also used in local elections around the world, as well as by some political parties (to elect internal leaders) and private associations. IRV is described in Robert's Rules of Order Newly Revised (under the name "preferential voting").

...
Wikipedia