The term black swan event refers to an incident of immense impact, which was not anticipated, and has a random probability of occurring. Only recognized in hindsight, individuals assume the risk of a black swan event when they invest in financial markets.
A black swan is a metaphor for something that is completely unexpected, since swans are normally white. The term is derived from the book The Black Swan, the Impact of the Highly Improbable, which was written by Nassim Nicholas Taleb. According to the author, a black swan event must possess the following characteristics:
The term grey swan was derived from this same concept, and is used to describe an incident of sizable impact, which can be anticipated, but has a relatively low probability of occurring.