Longman Dictionary of Contemporary English
The Collaborative International Dictionary
By-election \By"-e*lec"tion\, n. An election held by itself, not at the time of a general election.
Wiktionary
alt. a special election held to fill a political office that has become vacant between general elections. n. a special election held to fill a political office that has become vacant between general elections.
Wikipedia
By-elections (known as special elections in the United States) are used to fill elected offices that have become vacant between general elections.
In most cases these elections occur after the incumbent dies or resigns, but they also occur when the incumbent becomes ineligible to continue in office (because of a recall, ennoblement, criminal conviction, or failure to maintain a minimum attendance). Less commonly, these elections have been called when a constituency election is invalidated by voting irregularities.
In the United States, these contests have been called "special elections" because they do not always occur on Election Day like regular congressional elections. Despite their name, however, special elections to the U.S. House happen quite often. Furthermore, one published study shows that special elections are explained by the same factors as regular congressional elections.
Usage examples of "by-election".
A friend in the Commons told me that Thomas Fielden received a note from your wife yesterday, saying that you and the London Ladies Society mean to support him in the by-election.
Labour held on but the Tory vote surged and went on rising in the Leeds West by-election in July, where we worked to get Charlie Pannell elected.
He represented Great Britain in the 100 metres in the early sixties, and became the youngest member of the House of Commons when he won the by-election at Louth in 1969.
Before the 1958 by-election in the Manitoba constituency of Springfield, for example, Ottawa Tories were worried about the effects of a recent freight-rate increase.
You'll pick up votes in by-elections and eventually you'll come around and tell me I can add 'Emperor of Mars' to the list.
With the general public their influence is nil, as one can see by the votes in the by-elections, and the powerful hold they had on the press in the years 1935-9 has been completely broken.
As a party it won by-elections during the war but fared disastrously in the 1945 General Election when most of its twenty-three candidates, including Acland, lost their deposits against Labour opposition.
His - Paul Bethune's - party had recently picked up a couple of marginal seats in by-elections and were confident of taking Hoopwestern since 'the need for change' was in the air.
You fling your money about - ducks and drakes - you talk about marriage, although you know, or ought to know, that it is death to a sea-officer's career, at least until he is made post - you lead drunken parties at a Tory by-election - you come here and say you must have a ship.