A by-election (occasionally also spelled "bye-election", and known in the United States and the Philippines as a special election) is an election held to fill a political office that has become vacant between regularly scheduled elections. Usually this occurs when the incumbent has died or resigned, but it may also occur when the incumbent becomes ineligible to continue in office, for example because of a recall, ennoblement (colloquially known in the United Kingdom as being "kicked upstairs") or a sufficiently serious criminal conviction. By-elections have also been called as a result of a constituency election being invalidated due to voting irregularities.
Historically, members of some parliaments were required to seek re-election upon being appointed to a ministerial post. The subsequent by-elections were termed ministerial by-elections. These by-elections were usually a formality as they were normally, but not always, uncontested by opposition parties. The requirement for MPs to resign their seats and re-offer upon being appointed to Cabinet was done away with in most Westminster systems by the mid-20th century as an anachronism.