Definition:Delta-Ring

Definition
A delta-ring (which can conveniently be written $\delta$-ring) is a ring of sets which is closed under countable intersections.

That is, a ring of sets $\RR$ is a delta-ring :
 * $\displaystyle A_1, A_2, \ldots \in \RR \implies \bigcap_{n \mathop = 1}^\infty A_n \in \RR$