Black swans: Since assuming all swans are white, this is a metaphor to describe an event that comes as a surprise, which has a significant impact and that can only be explained in a proper manner after it has happened.
- A black swan comes as a surprise - A black swan has great effect - In retrospect we see that all data were available to predict a black swan, but no/few took into account the black swan.
Relevance: The one thing you can say with certainty is that the future scenario that you base your strategy on, is wrong. There is no "normal state" so you must take into account unexpected events in strategy planning. See also Scenarios.

The term was introduced by Nassim Nicholas Taleb in his book " Fooled By Randomness" in 2001, and made famous in the book “Black Swan” in 2007.