Definition:Sigma-Ring/Definition 1

Definition
A $\sigma$-ring is a ring of sets which is closed under countable unions.

That is, a ring of sets $\mathcal R$ is a $\sigma$-ring iff:
 * $\displaystyle A_1, A_2, \ldots \in \mathcal R \implies \bigcup_{n \mathop = 1}^\infty A_n \in \mathcal R$

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.