In a conversation with my dad about the collapse of civilization and society as we know it, he referenced the term "black swan event." After a quick discussion of what the term meant, I conducted further research (naturally starting with Wikipedia)....
The Black Swan theory (in Nassim Nicholas Taleb's version) refers to a large-impact, hard-to-predict, and rare event beyond the realm of normal expectations. Unlike the philosophical "black swan problem", the "Black Swan" theory (capitalized) refers only to events of large consequence and their dominant role in history. Black Swan events may also be called outliers.
The theory was described by Nassim Nicholas Taleb in his 2007 book The Black Swan. Taleb regards many scientific discoveries as "black swans" — undirected and unpredicted. He gives the rise of the Internet, the personal computer, World War I, and the September 11, 2001 attacks as examples of Black Swan events.
The term Black Swan comes from the assumption that 'All swans are white'. In that context, a black swan was a metaphor for something that could not exist. The 18th Century discovery of black swans in Australia metamorphosed the term to connote that the perceived impossibility actually came to pass.
Based on the author's criteria:
1. The event is a surprise.
2. The event has a major impact.
After the fact, it is usually the case that the event is rationalized by hindsight, as if it was expected to occur.
A few other articles discussing the significance of the Black Swan theory...
The Black Swan Theory of Chance
The Black Swan Theory of Being Unable to Predict Unexpected Events
(photo via here)