The term kurtosis refers to the degree to which a probability distribution has tails, regions of small probability extending high and/or low from the highest-probability distribution. A widely-used measure (due to Karl Pearson) is the fourth Pearson moment of the distribution. A formula:
Σ (Yi-m)4 kurtosis = ————————— Σ (Yi-m)2
This measure of the normal distribution is always 3, so sometimes 3 is subtracted, which is called the excess kurtosis.