Definition:Sigma-Ring/Definition 3

Definition
A $\sigma$-ring $\mathcal R$ is a system of sets with the following properties:

Also see

 * Equivalence of Definitions of Sigma-Ring

Linguistic Note
The $\sigma$ in $\sigma$-ring is the Greek letter sigma which equates to the letter s.

$\sigma$ stands for for somme, which is French for union.