Sigma

Sigma may refer to many things:

• Sigma (upper case Σ, lower case σ, alternative ς) is a letter in the Greek alphabet. See Sigma (letter).
• The upper-case letter Σ is used as a symbol for the summation operator forming a series in mathematics.
• The upper-case letter Σ is used as a symbol for a certain alphabet of a language, or of some other object dependent on an alphabet; expressed in mathematical terms, eg "the language defined by alphabet Σ={a,b,c} and the rules ..."
• The lower-case letter σ is used as a symbol for the standard deviation of a population in statistics.
• The lower-case letter σ is used as a symbol for the divisor function in number theory.
• The lower-case letter σ is used as a symbol for the Stefan-Boltzmann constant of radiation in physics.
• The name of the main villain from the video game series Mega Man X.