Sigma (Σ σ ς) is the 18th letter of the Greek alphabet and has a numeric value of 200. When used at the end of a word, the final form (ς) is used.
The upper-case letter Σ is used as a symbol for:
The summation operator forming a series in mathematics.
A certain alphabet, of a language, or someother object dependent on an alphabet; expressed in mathematical terms, eg "the language defined by alphabet Σ={a,b,c} and the rules ..."
