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Black Swan


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.


Reference: Oxford Press Dictonary of Economics, 5th edt.