\begin{definition}[Definition:Disjoint Events]
Let $A$ and $B$ be events in a probability space.
Then $A$ and $B$ are '''disjoint''' {{iff}}:
:$A \cap B = \O$
It follows by definition of probability measure that $A$ and $B$ are '''disjoint''' {{iff}}:
:$\map \Pr {A \cap B} = 0$
That is, two events are '''disjoint''' {{iff}} the probability of them both occurring in the same experiment is zero.
That is, {{iff}} $A$ and $B$ can't happen together.
\end{definition}