Definition:Sigma-Algebra/Definition 3

Definition
A $\sigma$-algebra $\mathcal R$ is a $\sigma$-ring with a unit.

Also see

 * Equivalence of Definitions of Sigma-Algebra

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.