by-election
noun
- special election to fill an empty position between general elections
Wiktionary
noun
Etymology: From by- (“side, secondary, ancillary”) + election.
- An election held to fill a political office that has become vacant between general elections.
“Coordinate term: general election”
“In 2015, he was selected as candidate for that year's by-election for the Oldham West and Royton seat. He won with 62% of the vote.”