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Investopedia explains 'Tail Risk'
When a portfolio of investments is put together, it is assumed that the distribution of returns will follow a normal pattern. Under this assumption, the probability that returns will move between the mean and three standard deviations, either positive or negative, is 99.97%. This means that the probability of returns moving more than three standard deviations beyond the mean is 0.03%, or virtually nil. However, the concept of tail risk suggests that the distribution is not normal, but skewed, and has fatter tails. The fatter tails increase the probability that an investment will move beyond three standard deviations.
Distributions that are characterized by fat tails are often seen when looking at hedge fund returns.
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