What is a Black Swan Event?

Black Swan

The Black Swan Theory or "Theory of Black Swan Events" is a metaphor that encapsulates the concept that The event is a surprise (to the observer) and has a major impact.

After the fact, the event is rationalized by hindsight.
Think of the economic crises and the Lehamn collapse.

The theory was developed by Nassim Nicholas Taleb to explain:

The disproportionate role of high-impact, hard to predict, and rare events that are beyond the realm of normal expectations in history, science, finance and technology

The non-computability of the probability of the consequential rare events using scientific methods (owing to the very nature of small probabilities)

The psychological biases that make people individually and collectively blind to uncertainty and unaware of the massive role of the rare event in historical affairs

How to identify a Black Swan?
You cannot because the event is a surprise (to the observer). The event has a major impact and after its first recording, the event is rationalized by hindsight, as if it could have been expected (e.g., the relevant data were available but not accounted for).

The Black Swan Impact of a Black Swan

For those interested we can recommend reading: