What is sigma?

Sigma is the eighteenth letter of the Greek alphabet. It has a numerical value of 200 in the Greek numeral system. In mathematics, sigma is used as a symbol for summation, indicating that a sequence of numbers should be added together. It is also used in statistics as a symbol for standard deviation, which is a measure of how spread out the data is. In chemistry, sigma bonds are the strongest type of covalent bond between atoms. Sigma is also used as a branding for companies and organizations, such as Sigma-Aldrich, a chemical and life science company, and Phi Beta Sigma, a fraternity.