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bus factor

/bʌs ˈfæktər/ - n.

1 [colloq.] Usually one, usually unaware, usually about to take leave.Keep. Punchy.This is the problem.

Working definition

2. The number of people who would have to be lost before a system or definition became unmaintainable; a lower number is a higher risk.

Promoted

See also

acting data stewardThe only person who knows what the field means, formally prevented from being the one who knows what the field means.
it runs on dave's laptopThe architecture diagram for a critical daily process, redrawn every time Dave takes a vacation.
tribal knowledgeThe real documentation, sharded across the people most likely to resign.