Black Swan events
Black Swan events are a metaphor that describes a situation or outcome that is outside the range of expected expectations. Events that are a surprise, have a major effect, and events that are rationalised post-hoc as if they could have been predicted: the data was there but nobody found it. The term, first used back in the 2nd Century, is a subjective term as every observer's red lines for what constitutes a "surprise" or "major" event can be different. It is often cited in studies of randomness and statistics.
The term found new prominence in 2007 with the release of a book by Nicolas Taleb named The Black Swan - The Impact of the Highly Improbable. The book was not without its academic critics, but other influential authors such as Daniel Kahneman cited Taleb's book as an influence on his own works. One of the main ideas of the book was that we should not to try to predict random events but to factor robustness into systems to cope with them.
Black Swan events come up in philosophy and logic as well; "All swans are white because I have never seen a black swan" being just one example of a fun argument you can have with all the family during a rainy weekend stuck in the house. They may also have a crossover with conspiracy theories. It is often said that conspiracy theories are "coping mechanisms" for unpredictable, major events.