A by-election, also known as a special election in the United States and the Philippines, a bye-election in Ireland, a bypoll in India, or a Zimni election (Urdu: ضمنی انتخاب, supplementary election) in Pakistan, is an election used to fill an office that has become vacant between general elections.

A vacancy may arise as a result of an incumbent dying or resigning, or when the incumbent becomes ineligible to continue in office (because of a recall, election or appointment to a prohibited dual mandate, criminal conviction, or failure to maintain a minimum attendance), or when an election is invalidated by voting irregularities. In some cases a vacancy may be filled without a by-election or the office may be left vacant.