What is what the sigma?

The term "sigma" can refer to several things depending on the context:

1. Greek Alphabet: Sigma (Σ, σ, ς) is the 18th letter of the Greek alphabet. The uppercase Σ and lowercase σ are commonly used in mathematics, science, and engineering.

2. Mathematics and Statistics:

   • Summation Notation: The uppercase Sigma (∑) is used to denote summation, meaning the addition of a sequence of numbers.
   • Standard Deviation: In statistics, lowercase sigma (σ) represents the standard deviation of a population, a measure of the amount of variation or dispersion in a set of values.

3. Six Sigma: A methodology and set of management techniques for process improvement that seeks to improve the quality of output by identifying and removing causes of defects and minimizing variability in manufacturing and business processes. It uses a set of quality management methods, mainly empirical, statistical methods, and creates a special infrastructure of people within the organization.

4. Sigma Factor: In molecular biology, a sigma factor is a protein needed for the initiation of transcription in bacteria. It is a bacterial transcription initiation factor that enables specific binding of RNA polymerase to gene promoters.

5. Finance: In finance, sigma is sometimes used to represent volatility or the standard deviation of returns of an asset.

6. Electricity and Robotics: In some contexts, Sigma can refer to a brand or product name, such as Sigma lenses for cameras or Sigma Aldrich chemicals.

Each instance of "sigma" is domain-specific, serving unique purposes in different fields.