A by-election, also known as a special election in the United States and the Philippines, a bye-election in Ireland, and a bypoll in India, is an election used to fill an office that has become vacant between general elections.

First-past-the-post voting is a plurality voting system wherein voters cast a vote for a single candidate, and the candidate with the most votes wins the election. Analogous systems for multi-winner contests are known as plurality block voting or "block voting" systems; both FPTP and block voting are "plurality" systems in that the winner needs only a plurality of the votes and not an absolute majority. The term first-past-the-post is a metaphor from horse racing of the plurality-voted candidate winning such a race; the electoral system is formally called single-member [district] plurality voting (SMP/SMDP) when used in single-member districts, and informally called choose-one voting in contrast to ranked voting or score voting.