Definitions from Wiktionary (sigma)

▸ noun:  The eighteenth letter of the Classical and Modern Greek alphabets (Σ, σ), the twentieth letter of Old and Ancient.
▸ noun:  (mathematics) The symbol Σ, used to indicate summation of a set or series.
▸ noun:  (statistics) The symbol σ, used to indicate one standard deviation from the mean, particularly in a normal distribution.
▸ noun:  (Internet slang) A sigma male (a self-reliant and assertive male).

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