Something rare, something you've never considered a possibility...these events are, what Nassim Taleb calls 'fat tails' or 'black swans,' and he is convinced that they take place far more frequently than most human beings are willing to...
...contemplate. The normal distribution idea, pioneered by Carl Friedrich Gauss in the early 19th century, does not work so well when the nostrums of normalcy must be stretched to include unforeseen wars, pandemics, terror attacks, tsunamis, the madness of crowds and the bursting of bubbles.
More precise (What are fat tails?):
To comprehend what fat tail is, think of a bell curve, the line on a statistician's chart that reflects "normal distribution." It is tall and wide in the middle - where most people and things being studied almost always tend to be - and drops and flattens out at the bottom, where fewer are, making a shape on a graph resembling a bell. The extremities at the bottom left and right are called the tails; when they balloon instead of nearly vanishing as expected, the tails have been designated "heavy" OR "fat."