Event Definition
An event is a certain subset of a probability space. Events are therefore collections of outcomes on which probabilities have been assigned. In general, an event is any subset of a sample space (including possibly an empty set).
A sure event would mean it has a 100% probability of occurring whereas in contrast, an impossible event has zero probability of occurring. For example, a possible event when a single six-sided die is rolled is {2, 3}. That is, rolling the die could yield a 2 or a 3.
Related Definitions
Sources
“Event.” From Wolfram MathWorld, mathworld.wolfram.com/Event.html.