A black swan, also known as "black swan theory" or "black swan events", is is a metaphor that describes an event that comes as a surprise, has a major effect, and is often inappropriately rationalized after the fact with the benefit of hindsight.
The background for the name black swan was that the black swan was considered a creature that did not exists, but when in fact the creature was discovered in the wild it falsified all previous assumptions. The importance of the metaphor lies in its analogy to the fragility of any system of thought. A set of conclusions is potentially undone once any of its fundamental postulates is disproved. In this case, the observation of a single black swan would be the undoing of the logic of any system of thought, as well as any reasoning that followed from that underlying logic.
The phrase "black swan" derives from a Latin expression; its oldest known occurrence is from the 2nd-century Roman poet Juvenal's characterization of something being "rara avis in terris nigroque simillima cygno" ("a rare bird in the lands and very much like a black swan").
Juvenal's phrase was a common expression in 16th century London as a statement of impossibility. The London expression derives from the Old World presumption that all swans must be white because all historical records of swans reported that they had white feathers. In that context, a black swan was impossible or at least nonexistent.
However, in 1697, Dutch explorers led by Willem de Vlamingh became the first Europeans to see black swans, in Western Australia. The term subsequently metamorphosed to connote the idea that a perceived impossibility might later be disproven.