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.

• A classification of quantum particle - see baryon.

• sigma₁ and sigma₂ are cell receptors in biochemistry and pharmacology.

• The lower-case letter σ is used as a symbol for the Sigma factor of RNA polymerase in biology.

• Sigma is the name of the main villain from the video game series Mega Man X.

• Sigma Corporation is a manufacturer of cameras, lenses and camera accessories.

• A model of handgun manufactured by Smith and Wesson.

• The Mitsubishi Motors Corporation car known as the Mitsubishi Sigma (variant of world-wide released Mitsubishi Galant).