The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sigma
    n 1: the 18th letter of the Greek alphabet

The Free On-line Dictionary of Computing (19 January 2023):

SIGMA

   A scientific visual programming environment from NASA.

   (http://fi-www.arc.nasa.gov/fia/projects/sigma/).