What's A Blackswan Event?

You may have heard the term “Black Swan Event“, particularly recently.  But what exactly is a black swan event and what can, and should, we learn from them?
The notion of black swan events was first set forth by Nassim Nicholas Taleb. Taleb, a finance professor and former Wall Street trader, wrote about this concept in his 2001 book Fooled by Randomness which concerned financial events. In a later work, The Black Swan, Taleb extends the metaphor to events outside the financial markets.  He defines three attributes that are common to all black swan events:

1. The event is unpredictable (to the observer)
2. The event has widespread ramifications
3. After the event has occurred, people will assert that it was indeed explainable and predictable (hindsight bias).

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