The symbol σ is a Greek letter, which is expressed in English as Sigma (capital σ, lowercase σ,) and transliterated in Chinese as Sigma, which is the18th Greek letter. Sigma is a statistical unit, which is used to measure the standard error of a total. In chemistry, it is also used to represent a valence bond, σ bond. The extended data с of Cyrillic alphabet and S of Latin alphabet are all evolved from Sigma. Capital σ represents the sum sign in mathematics, which is a common symbol in mathematics. It is mainly used to find the sum of multiple items, which is expressed by ∑ (also called the sum sign). The symbol "σ" was first used by Euler (1707- 1783) in 1755. This symbol comes from the Greek prefix (addition), and "σ" is the capital of σ.

For business or manufacturing process, σ value can measure the ability of the process to complete defect-free operation, and it is a yardstick to show the good state of the process. The lower the σ value, the better the process conditions. The defect rate of general enterprises is about 3 to 4 horses. If the enterprise constantly pursues quality improvement and reaches the level of 6 sigma, the performance will almost perfectly meet the requirements of customers. Six Sigma developed from a total quality management method to an efficient enterprise process design, improvement and optimization technology in the mid-1990s, and provided a series of new product development tools which are also suitable for design, production and service.