Definitions for: Sigma

[n] the 18th letter of the Greek alphabet

Webster (1913) Definition: Sig"ma, n.; pl. Sigmas. [L., from Gr. ???, ???.]
The Greek letter [Sigma], [sigma], or [sigmat] (English S, or
s). It originally had the form of the English C.

See Also: alphabetic character, Greek alphabet, letter, letter of the alphabet