Instant Run-Off Voting (IRV, also known as the Alternative Vote) is a voting system used used to elect a single member by preference.
It is called instant run off because is is designed to simulate multiple rounds of run-off elections.
As only one seat or position is at available parties will normally only put up one candidate, but they can put up more. Voters rank candidates in order of preference by marking the ballot paper 1, 2, 3 etc.
If in the first count no candidate has more than 50% of the vote then the candidate with the least votes is eliminated and that candidate's votes are redistributed at full value to their second choices; this process continues until someone has a majority. This ensures that a majority of voters actually supports the winning candidate.
Why it's fairerEdit
The primary reason why this system is better than the alternatives is because it avoids the so-called spoiler effect. Spoiler effect is a condition where appearance of a smaller candidate on the election actually harms the bigger candidate they're close to. By getting some of the votes from a bigger candidate people still support, they let the opposing candidate(s) get a first place easier.
For a real-life example, in the 2000 US Presidential Election, Ralph Nader (a liberal independent) was running against Al Gore and George W. Bush. In Florida (the most important swing state in that election) Bush defeated Gore with 537 votes. Ralph Nader, on the other hand, had 97,421 votes, with exit polls showing that more of the candidates would otherwise vote for Al Gore if Nader didn't run. Which, in turn, could have led to a significantly better end of the story. With IRV, the candidates who supported Gore would have picked him as his second choice, still letting him win Florida's electoral votes.
It avoids tactical voting and squeeze voting where supporters of third parties are told their votes will be wasted.
It also avoids an unpopular candidate winning on a split vote. If more than 50% of those who vote in a seat put a particular candidate last then that candidate will not win.
Differences between Instant Run-Off voting and proportional votingEdit
IRV is not a proportional system. Though a parliamentary party must gain at least a preferential vote from at least 50% of the voting population in a majority of seats to gain a majority in Parliament. So with Instant Run-off voting you know that more than 25% of the voters didn't hate the Government. But they could in theory have little more than 50% support in 50% of the seats, - so very undemocratic results are still possible.