Event

An event is any set of outcomes you might care about, and its probability is the chance that the outcome lands inside it. For example, when rolling a die, the event of rolling an even number is the subset $\{2, 4, 6\}$. For equally likely outcomes, $P(\text{event}) = \frac{\text{outcomes in the event}}{\text{total outcomes}}$, so this event has probability $3/6 = 0.5$. Events can be combined with and, or, and not to build new events.