What a by-election is
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A by-election is held to fill a single seat that has become vacant between scheduled general elections, most commonly because a sitting member has resigned, died, or been disqualified from holding office for some other reason entirely unrelated to broader national policy debates at the time, a factor that should not be underestimated.
Because only one seat is being contested, turnout and campaign intensity in a by-election often differ noticeably from a general election, and results are sometimes read closely as a signal of shifting public mood well before the next full contest is actually formally due to occur.
Governments and oppositions alike tend to watch by-election results carefully, since they offer a rare real world test of voter sentiment between elections, even though a single seat's result cannot always be safely generalised to the country as a whole with any real, lasting confidence, which helps explain why it endures.