\begin{definition}[Definition:Dynkin System]
Let $X$ be a set, and let $\DD \subseteq \powerset X$ be a collection of subsets of $X$.
Then $\DD$ is called a '''Dynkin system (on $X$)''' {{iff}} it satisfies the following conditions:
:$(1): \quad X \in \DD$
:$(2): \quad \forall D \in \DD: X \setminus D \in \DD$
:$(3): \quad$ For all pairwise disjoint sequences $\sequence {D_n}_{n \mathop \in \N}$ in $\DD$, $\ds \bigcup_{n \mathop \in \N} D_n \in \DD$
\end{definition}