black swan

An Australian black swan

Meaning

Cygnus atratus, an Australian swan whose feathers are black.
Something believed impossible or not to exist, of which an example is subsequently found.
A rare and hard-to-predict event with major consequences.

A calque, equivalent to black + swan. Roman satirist Juvenal wrote in AD 82 of rāra avis in terris nigroque simillima cygno, creating a durable metaphor and expression. In the sense “unforeseen event” popularized by Nassim Nicholas Taleb in a 2007 book of the same name.

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