Ranked voting system

Preferential voting or ranked-choice voting describes certain voting systems in which voters rank outcomes in a hierarchy on the ordinal scale (ordinal voting systems). When choosing between more than two options, preferential ballots collect more information from voters than first-past-the-post voting (also called plurality voting). This does not mean that preferential voting is the best system; Arrow's impossibility theorem proves that no preferential method can simultaneously obtain all properties desirable in a voting system. There is, accordingly, no consensus among academics or public servants as to the best electoral system.

There are many types of preferential voting, with several used in governmental elections: Instant-runoff voting is employed in Australia at the state and federal levels, in Ireland for its presidential elections, and by some cities in the United States, United Kingdom, and New Zealand. The single transferable vote is used for national elections in the Republic of Ireland and Malta, the Australian Senate, for regional and local elections in Northern Ireland, for all local elections in Scotland, and for some local elections in New Zealand and the United States. Contingent vote, Supplementary vote, and Borda count are also used in a few locations. Condorcet methods have found more use among private organizations and minor parties.

The other major branch of voting systems are cardinal voting systems, where candidates are rated, rather than ranked.

There are many preferential voting systems, so it is sometimes difficult to distinguish between them.

Selection of the Condorcet winner is generally considered by psephologists as the ideal election outcome for a ranked system, so "Condorcet efficiency" is important when evaluating different methods of preferential voting. The Condorcet winner is the one that would win every two-way contest against every other alternative.

...
Wikipedia