Outsized impact of rare, hard-to-predict events. Reframed risk thinking after 2008 and is now embedded in everyday strategic-planning vocabulary.
From Wikipedia
The black swan theory or theory of black swan events is a metaphor that describes an event that comes as a surprise, has a major effect, and is often inappropriately rationalized after the fact with the benefit of hindsight. The term arose from a Latin expression which was based on the presumption that black swans did not exist. The expression was used in the original manner until around 1697 when Dutch mariners saw black swans living in Australia. After this, the term was reinterpreted to mean an unforeseen and consequential event.