A special election held in a single constituency to fill the place of the constituency's MP if they have died, resigned or moved to another position.
A byelection is a small election held for one or two seats in a parliamentary legislature. In a general election, all seats are to be contested, but in the event that a member of parliament resigns or dies or for some other reason vacates the seat, they have a special election, a byelection, to fill that one seat.