Sigma ( SIG-mə; uppercase Σ, lowercase σ, lowercase in word-final position ς; Greek: σίγμα) is the eighteenth letter of the Greek alphabet. When it is used at the end of a letter-case word (one that does not use all caps), the final form (ς) is used. In Ὀδυσσεύς (Odysseus), for example, the two lowercase sigmas (σ) in the center of the name are distinct from the word-final sigma (ς) at the end.
In the system of Greek numerals, sigma has a value of 200. In general mathematics, Σ is used as an operator for summation. The Latin letter S derives from sigma while the Cyrillic letter Es (С) derives from a lunate form of this letter.
