# Definition:Delta-Ring

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 if and only if:
$\ds A_1, A_2, \ldots \in \RR \implies \bigcap_{n \mathop = 1}^\infty A_n \in \RR$