The event E of a sample space SS is said to have occurred if the outcome ωω of the experiment is such that ω∈E. If the outcome ω is such that ω∉E, we say that the event EE has not occurred.
E.g.: Consider the experiment of drawing a card from a pack of cards. Let E denote the event “drawing of an ace”. If actually an ace has been drawn, we say that event E has occurred.