The Black Swan Theory or Theory of Black Swan Events is a metaphor that encapsulates the concept that the event is a surprise (to the observer) and has a major impact. After the fact, the event is rationalized by hindsight.
The theory was developed by Nassim Nicholas Taleb to explain:
1. The disproportionate role of high-impact, 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 the consequential rare events using scientific methods (owing to the very nature of small probabilities)
3. The psychological biases that make people individually and collectively blind to uncertainty and unaware of the massive role of the rare event in historical affairs
Unlike the earlier philosophical "black swan problem", the "Black Swan Theory" (capitalized) refers only to unexpected events of large magnitude and consequence and their dominant role in history.