What is a black swan event?

From litefinance.com

The term ‘black swan’ originates from Latin in the form of an expression "a rare bird on Earth, like a black swan." For a long time in Europe, people believed that swans other than white did not exist. However, in the 17th century, a population of black swans was discovered in Western Australia, but the expression is still used to describe something unlikely. So, what a black swan event is? The term ‘black swan’ is used to describe economic, political and social accidents that are inevitable and impossible to predict. This is exactly what Nassim Nicholas Taleb, a former Wall Street trader and the author of ... (full story)