black swan

noun

1. an Australian swan, Cygnus atratus, having mostly black plumage and a red bill.
2. an occurrence or phenomenon that comes as a surprise because it was not predicted or was hard to predict (often used attributively): a disastrous black-swan event that only appeared obvious in hindsight.

noun

1. a large Australian swan, Cygnus atratus, that has a black plumage and red bill
2. a phenomenon that occurs even though it had been thought to be impossible