Hypertext Webster Gateway: "sigma"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
Sigma \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.
From WordNet (r) 1.7 (wn)
sigma
n : the 18th letter of the Greek alphabet
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