A metaphoric name for a rare event. Nassem Nicholas Taleb, who put forward the Black Swan Theory, defined the Black Swan event as an unexpected, unpredicted event that has a major impact and is often rationalized ex post. An example is the September 11 terrorist attacks in the US in 2001: standard finance theory did not predict the stock market crash that followed the attacks, because such an event had not been observed before and was, therefore, outside the existing models.