black swans

Noun

black swan (plural black swans)

1. Cygnus atratus, a swan with black plumage and a red bill which is endemic to Australia.
   • Mankind were wrong, it seems, in concluding that all swans were white: are we also wrong, when we conclude that all men's heads grow above their shoulders, and never below, in spite of the conflicting testimony of the naturalist Pliny? As there were black swans, though civilised people had existed...1843, John Stuart Mill, “Of the Ground of Induction”, in A System of Logic, Ratiocinative and Inductive, being a Connected View of the Principles of Evidence, and the Methods of Scientific...
2. (figuratively) Something believed impossible or not to exist, especially if an example is subsequently found; also, something extremely rare; a rara avis.
   • I do but ſpeake in the perſon of Demetrius, & vnder Hippolita ſhadovv vvhat I intend to the rare, and neuer enough vvondred at Moſpa, the black ſvvan of beauty, & madg-hovvled of admiration.1605 (date written), Iohn Day [i.e., John Day], The Ile of Guls. […], London: […] [John Trundle], and are to bee sold by Iohn Hodgets […], published 1606, →OCLC, signature [D4], verso

   Synonyms:

   rare bird, rarity, unicorn, hen's tooth, lightning in a bottle, one in a million, rara avis, rocking horse shit

3. (also, attributive, figuratively, specifically) A rare and hard-to-predict event with major consequences.
   • A small number of Black Swans explain almost everything in our world, from the success of ideas and religions, to the dynamics of historical events, to elements of our own personal lives. […] Fads, epidemics, fashion, ideas, the emergence of art genres and schools. All follow these Black Swan...2007, Nassim Nicholas Taleb, “Prologue”, in The Black Swan: The Impact of the Highly Improbable, New York, N.Y.: Random House, →ISBN, page xviii