# Definition:Sigma-Algebra/Definition 3

## Definition

A $\sigma$-algebra $\Sigma$ is a $\sigma$-ring with a unit.

## Linguistic Note

The $\sigma$ in $\sigma$-algebra is the Greek letter sigma which equates to the letter s.

$\sigma$ stands for for somme, which is French for union, and also summe, which is German for union.