What is pi-system?

A $\pi$-system is a family stable under finite intersections. A $\lambda$-system (Dynkin class) is a family $\mathcal D$ with: $X \in \mathcal D$; $A, B \in \mathcal D$, $A \subseteq B$ $\Rightarrow$ $B \setminus A \in \mathcal D$; and $A_n \uparrow A$, $A_n \in \mathcal D$ $\Rightarrow$ $A \in \mathcal D$.