What is Sigma?

Legal Definition
Sigma (upper-case Σ, lower-case σ, lower-case in word-final position ς; Greek: σίγμα) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. When used at the end of a word (when the word is not all caps), the final form (ς) is used, e.g. Ὀδυσσεύς (Odysseus); note the two sigmas in the center of the name, and the word-final sigma at the end.
-- Wikipedia
Legal Definition
1. The Greek symbol for sigma that represents a sum ot total operation.2. See also six sigma.