Sigma
| Greek letters | |
| Α α Alpha | Β β Beta |
| Γ γ Gamma | Δ δ Delta |
| Ε ε Epsilon | Ϝ ϝ Digamma |
| Ζ ζ Zeta | Η η Eta |
| Θ θ Theta | Ι ι Iota |
| Κ κ Kappa | Λ λ Lambda |
| Μ μ Mu | Ν ν Nu |
| Ξ ξ Xi | Ο ο Omicron |
| Π π Pi | Ϻ ϻ San |
| Ϙ ϙ Qoppa | Ρ ρ Rho |
| Σ σ Sigma | Τ τ Tau |
| Υ υ Upsilon | Φ φ Phi |
| Χ χ Chi | Ψ ψ Psi |
| Ω ω Omega | Ϡ ϡ Sampi |
| Greek alphabet | |
Sigma (upper case Σ, lower case σ, alternative ς) is the 18th letter of the Greek alphabet. In the system of Greek numerals it has a value of 200. When used at the end of a word, the final form (ς) is used.
The upper-case letter Σ is used as a symbol for:
- The summation operator forming a series in mathematics.
- 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 standard deviation of a population in statistics.
- The divisor function in number theory.
- The Stefan-Boltzmann constant of radiation in physics.