Skewness

n. obliqueness, quality of being oblique

In probability theory and statistics, skewness is a measure of the asymmetry of the probability distribution of a real-valued random variable.
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Noun
1. an oblique or slanting asymmetry
(synonym) lopsidedness
(hypernym) asymmetry, imbalance

Negative skewness means there is a substantial probability of a big negative return. Positive skewness means that there is a greater than normal probability of a big positive return.

Skewness (this term was first used by Pearson, 1895) measures the deviation of the distribution from symmetry. If the skewness is clearly different from 0, then that distribution is asymmetrical , while normal distributions are perfectly symmetrical .
Skewness = n*M3/[(n-1)*(n-2)* 3]
where
M3 is equal to: (xi-Meanx)3
3 is the standard deviation (sigma) raised to the third power
n is the valid number of cases.
See also, Descriptive Statistics Overview .