A black swan event is an unprecedented, unpredictable event that has a widespread effect on the market. Often, a black swan event is rationalized in hindsight.
One of the most famous black swan events took place in September 1998 with the collapse of the $4.6 billion hedge fund, Long-Term Capital Management. Long-Term Capital was heavily invested in Russian bonds, and when the Russian government defaulted, it caused a ripple effect through the global markets and Long-Term Capital collapsed. There was no way that LTC could have predicted this event and its aftereffects.