Definition:Event/Occurrence/Certainty

Definition
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.

Also see

 * Definition:Impossible Event