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.

The reinterpreted theory was articulated by Nassim Nicholas Taleb, starting in 2001, to explain:

1. The disproportionate role of high-profile, hard-to-predict, and rare events that are beyond the realm of normal expectations in history, science, finance, and technology.
2. The non-computability of the probability of consequential rare events using scientific methods (owing to the very nature of small probabilities).
3. The psychological biases that blind people, both individually and collectively, to uncertainty and to the substantial role of rare events in historical affairs.

In his 2010 book, Taleb defines the term as an event with two characteristics: first, it is so rare and outside the realm of expectations that it is unpredictable; second, its consequences are extreme—either beneficial or catastrophic—though usually only the catastrophic Black Swan events attract attention. Definitionally, Taleb considers black swans to be in the eye of the beholder and warns that objectively defining a black swan in a way "invariant in the eyes of all observers" would be erroneous. Taleb provides the example of the 9/11 attacks, which were a black swan for many, but not for its planners and perpetrators.

Taleb's "black swan theory" (which differs from the earlier philosophical versions of the problem) refers only to statistically unexpected events of large magnitude and consequence and their dominant role in history. Such events, considered extreme outliers, collectively play vastly larger roles than regular occurrences.^: xxi More technically, in the scientific monograph "Silent Risk", Taleb mathematically defines the black swan problem as "stemming from the use of degenerate metaprobability".