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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.
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Origin of black swan

(def 2) from the belief that all swans were white, before the first sighting of Australian black swans
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for black swan

black swan

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
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Word Origin for black swan

sense 2 from the fact all swans were thought to be white until black swans were discovered in Australia
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012