\begin{definition}[Definition:Set Intersection/Countable Intersection]
Let $\mathbb S$ be a set of sets.
Let $\sequence {S_n}_{n \mathop \in \N}$ be a sequence in $\mathbb S$.
Let $S$ be the intersection of $\sequence {S_n}_{n \mathop \in \N}$:
:$\ds S = \bigcap_{n \mathop \in \N} S_n$
Then $S$ is a '''countable intersection''' of sets in $\mathbb S$.
\end{definition}