Definition of fat tail:
(n) :
The relatively high probability of a relatively extreme outcome.
(n) :
(statistics, probability) A tail of a probability distribution with significantly higher kurtosis than a normal distribution (which has kurtosis = 0).
(n) :
(statistics, probability) A tail of a probability distribution with infinite variance.
(n) :
A mass of fat stored in the rump of a beast as an adaptation to heat and harsh environmental conditions or selection by man.