Sigma

Sigma (uppercase/lowercase Σ σ ς) is the 18th letter of the Greek alphabet. It is used to represent the "s" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 235. Letters that came from it include the Roman S and Cyrillic С. 
 Mathematics 
The uppercase Σ is the shorthand symbol for summation (the symbol is also known as the summation operator). It is often used to condense Taylor series and to write out trigonometric such as sine, cosine, and tangent as series.

The lowercase σ is used to represent population standard deviation in statistics, and permutation function in abstract algebra.

 Related page 
 Stigma (letter)

 References 

Category:Greek alphabet