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. By-elections are held in most nations that elect their parliaments through single-member constituencies, whether with or without a runoff round. This includes most Commonwealth countries, such as the United Kingdom, Canada, Australia, and New Zealand, as well as non-Commonwealth countries such as France and Italy.

In the United Kingdom, a parliamentary by-election happens when a seat in the House of Commons becomes vacant between general elections. In New Zealand, a by-election is held to fill a vacant electorate seat, for example, if a member of Parliament resigns or dies. By-elections can also be important if a minority party needs to gain one or more seats to gain official party status or the balance of power in a minority or coalition situation.

To contest a by-election, a candidate must be a citizen of the country, enrolled to vote, and nominated by two people enrolled in the electorate where the by-election is being held. A candidate can stand on behalf of a political party or as an independent. During a by-election, voting places open across the electorate, and only people enrolled in the electorate can vote for their preferred candidate. When the voting period ends, votes are counted, and the final results are published. The candidate with the most votes becomes the elected member of Parliament for that electorate.