\begin{definition}[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''' {{iff}}:
:$\ds A_1, A_2, \ldots \in \RR \implies \bigcap_{n \mathop = 1}^\infty A_n \in \RR$
Category:Definitions/Set Systems
\end{definition}