Sigma (upper case Σ, lower case σ, alternative ς) is the 18th 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, and the word is not upper case, the final form (ς) is used.

In early forms of Greek writing and in the Middle Ages, the "lunate sigma" that looks like the letter C was often used. It can still sometimes be seen in inscriptions in Greek Orthodox churches, for example, where, for example, the whole of God's creation may be denoted by the word ΚΟCΜΟC (cosmos), which in the modern form of writing Greek would be ΚΟΣΜΟΣ.

See also

• Sigma – for other meanings.