WebSigma (/ ˈ s ɪ ɡ m ə / SIG-mə; uppercase Σ, lowercase σ, lowercase 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.In general mathematics, uppercase Σ is used as an operator for summation.When used at the end of a letter-case word (one that does not use all caps), the final form (ς) is ...