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 $$\mathcal R$$ is a delta-ring iff:
 * $$A_1, A_2, \ldots \in \mathcal R \implies \bigcap_{n=1}^\infty A_n \in \mathcal R$$