A by-election or bye-election is a type of special election. These elections are held in between general elections. General elections are elections for all members of an elected body. By-elections are elections for just some members. A by-election could be needed if a member dies or resigns while in political office. They are also used if a person is removed from office for some reason, for example if he/she is guilty of a crime or is not qualified to be a member anymore.
In the 1980s, all 12 of the Unionist MPs in the British House of Commons resigned and all of the elections happened on the same day. Although unusual, these were still only by-elections.