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.
