The term gray swan event refers to an incident of sizable impact, which can be anticipated, but has a relatively low probability of occurring. Often recognized in hindsight, individuals assume the risk of a gray swan event when they invest in financial markets.
A gray swan is a metaphor for something that is unexpected to a certain degree, since swans normally are white. The term is derived from the book The Black Swan, the Impact of the Highly Improbable, which was written by Nassim Nicholas Taleb. The difference between these two types of events is subtle:
To be labeled as a gray swan event, it must also have sizable impact on a financial market, or individual security.