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 200. Letters that came from it include the Roman S and Cyrillic С.
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 functions such as sine, cosine, and tangent as series.