Black Swan
Etymology
The Roman poet Juvenal called something impossible "a rare bird, like a black swan" (Latin "rara avis"), since Europeans had only ever seen white ones — until black swans were found in Australia in 1697, overturning the certainty. The financial writer Nassim Nicholas Taleb revived the image in his 2007 book "The Black Swan" for events that are rare, hugely consequential, and only explicable after the fact. The term folds its own history of a shattered assumption into its meaning.
noun phrase · /BLAK SWON/
A rare, high-impact event that is unpredictable beforehand but rationalized as obvious in hindsight.
What it names
For the outlier shock that models miss — and that everyone later claims they should have seen coming.
Example
The pandemic was a black swan that no one’s risk model had priced in.
Related idea
A black swan is unforeseeable before it strikes; a canary in the coal mine is an early warning you can heed in advance — hindsight-only versus a signal you can act on.