black swan

American  

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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black swan British  

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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Etymology

Origin of black swan

black swan ( def. 2 ) from the belief that all swans were white, before the first sighting of Australian black swans

Example Sentences

Although they may not come to pass, there are plenty of potential black swan events that could be lurking in the back half of the 2020s.

From Barron's • Feb. 26, 2026

In the words of one central-bank manager, he quoted, the precious metal is the “ultimate protection against black swan tail risk events”.

From MarketWatch • Nov. 26, 2025

That implies that barring a black swan event, the current easing cycle is close to done.

From The Wall Street Journal • Oct. 10, 2025

The hypothesized one-two punch of earthquakes was probably “not just a black swan chance-in-a-million,” Goldfinger said.

From Los Angeles Times • Oct. 7, 2025

The serving men brought out a heron stuffed with figs, veal cutlets blanched with almond milk, creamed herring, candied onions, foul-smelling cheeses, plates of snails and sweetbreads, and a black swan in her plumage.

From "A Dance with Dragons" by George R. R. Martin