Black Swan

An event or occurrence that deviates beyond what is normally expected of a situation and that would be extremely difficult to predict. This term was popularized by Nassim Nicholas Taleb, a finance professor and former Wall Street trader.

Black swan events are typically random and unexpected. For example, the previously successful hedge fund Long Term Capital Management (LTCM) was driven into the ground as a result of the ripple effect caused by the Russian government's debt default. The Russian government's default represents a black swan event because none of LTCM's computer models could have predicted this event and its subsequent effects.