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 is based on an ancient Roman saying expressing the European presumption that black swans did not exist until Dutch mariners saw them in Australia in 1697, and the term was then reinterpreted to mean an unforeseen and consequential event.

Nassim Nicholas Taleb is a Lebanese-American essayist, mathematical statistician, former option trader, risk analyst, and aphorist. His work concerns problems of randomness, probability, and uncertainty.