A Black Swan is an event with the following characteristics:

  1. It’s unpredictable
  2. It carries an extreme impact
  3. It seems predictable after the fact (and people will find explanations that seem logical in hindsight, but weren’t obvious before the fact)

This makes us overconfident in our ability to predict the future. World War I, 9-11 and the rise of the Internet, are all examples of Black Swans.

More:
The Black Swan (book) by Nassim Taleb
Thinking, Fast and Slow (book) by Daniel Kahneman