Definition of: sigma

(sig′mə) noun
1. The 18th letter in the Greek alphabet, written Σ (capital), σ (small initial), or ς (small final): corresponding to English s in so. As a numeral it denotes 200.
2. Math. The symbol signifying that the sum is to be taken of a series or sequence following.
3. Something shaped like a sigma. [<Gk. sigma the letter s]