sigma
The eighteenth letter of the Classical and Modern Greek alphabets (Σ, σ), the twentieth letter of Old and Ancient.
Other meanings
The symbol Σ, used to indicate summation of a set or series.
The symbol σ, used to indicate one standard deviation from the mean, particularly in a normal distribution.
Where this comes from
This definition is adapted from “sigma” on Wiktionary, by its contributors, published under CC BY-SA 4.0. This page is published under the same licence.