Taleb's definition of a "Black Swan":

partial quote from:

https://www.cnn.com/2018/10/22/politics/citizen-cnn-takeaways/index.html

Here's Taleb's own explanation -- from an excerpt of his 2007 book -- of what makes a black swan moment:

"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."