A by-election occurs when a seat in the House of Commons becomes vacant during the lifetime of a Parliament (ie between general elections), because the sitting MP dies, resigns (by applying for the Chiltern Hundreds or the Manor of Northstead), is elevated to the peerage or becomes ineligible to sit for some other reason.
Further information on this subject can be found from the following links.
By-elections since the last general election
About Parliament: By-elections
Library Briefing Paper: Resignation from the House of Commons
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