A by-election or bye-election is a special election held to fill a political office when the incumbent has died or resigned.
By-elections are held in most nations that elect their parliaments through single member contituencies, whether with or without a runoff round. This includes most Commonwealth countries, such as the United Kingdom and Canada, as well as France. In the United States they are called "special elections", and are held when a seat in the House of Representatives or state legislature is vacant and there is a long period (typically six months) until the next regular election. The Republic of Ireland holds by-elections despite electing members in multi-member constituencies by the Single Transferrable Vote - the alternative would be to recount ballots as in Tasmania.
The vast majority of by-elections are unimportant. The governing party normally has a solid cushion so that losing a handful of seats would not affect their position. Because of their inability to greatly affect the governance of the nation, voters feel freer to elect smaller fringe parties. Parties on both the far right and the far left tend to do better in by-elections than in general elections.
However, by-elections can become crucial when the ruling party has only a small margin. In parliamentary systems, party discipline is strong enough so that the one common scenario for a vote of no confidence to occur is after the governing party loses enough by-elections to become a minority government.
By-elections can also be important if a minority party needs to gain one or more seats in order to gain official party status or the balance of power in a minority or coalition situation. For example, Andrea Horwath's win in an Ontario provincial by-election in 2004 allowed the Ontario NDP to regain official party status, with important results in terms of parliamentary privileges and funding.
Party leaders and media commentators often point to by-election victories as important signals, but very often by-elections hinge far more on local issues and the charisma of the candidates than on national issues or how the voters feel about the governing party.
