A black swan is a seemingly improbable or unforseeable event with dramatic consequences.
Nassim Taleb gets credit for molding the black swan (a real type of bird) into a financial/economic term. Fool Jack Uldrich gave this synopsis in a 2007 book review of Taleb's The Black Swan: The Impact of the Highly Improbable: "The book takes its title from the story that says prior to the discovery of the Australian back swan, everyone in the world assumed all swans were white. The importance of the black swan - something that falls outside the realm of regular expectations -- is that a single such sighting can invalidate a general statement (i.e. "All swans are white.")."