Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

sigma
     n : the 18th letter of the Greek alphabet

Source : Free On-Line Dictionary of Computing

SIGMA
     
        A scientific {visual programming} environment from NASA.
     
        {(http://fi-www.arc.nasa.gov/fia/projects/sigma/)}.