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]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006):

SIGMA
       Software Industrialized Generator and Maintenance Aids system
(MITI)