Fat tails
The property of market returns whereby very large moves happen far more often than the normal curve predicts.
How to read the number
One practical conclusion follows: a rare event in market data is part of the distribution rather than a defect of the sample, and what has to be planned for is its size, not only its probability.
When the metric lies
Any risk measure derived from the normal distribution understates the loss in the tail, and the calmer the history looked, the larger that understatement is.
Also known as: heavy tails