black swan - definition and synonyms

noun [countable] 

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/ˌblæk ˈswɒn/
  1. an event that is extremely rare and unexpected but has very significant consequences

    The problem with black swan events is that very often they are never repeated.

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Word story
When Dutch explorer Willem de Vlamingh first saw black swans in Western Australia in the 17th century, Europeans believed that all swans were white, so a black swan seemed an impossibility. The term came to be used for an event that happens even though it seems impossible.