What is a Black Swan?

In The Black Swan: The Impact of the Highly Improbable, Taleb defined a black swan as

β€œan event with the following three attributes. First, it is an outlier, as it lies outside the realm of regular expectations, because nothing in the past can convincingly point to its possibility. Second, it carries an extreme impact…. Third, in spite of its outlier status, human nature makes us concoct explanations for its occurrence after the fact, making it explainable and predictable.”