\begin{definition}[Definition:Event/Occurrence/Certainty]
Let the probability space of an experiment $\EE$ be $\struct {\Omega, \Sigma, \Pr}$.
Let $A \in \Sigma$ be an event of $\EE$ whose probability of occurring is equal to $1$.
Then $A$ is described as '''certain'''.
That is, it is a '''certainty''' that $A$ occurs.
\end{definition}