**Probability** describes how likely something is to happen. In probability, an **event** is the single result of a trial. An **outcome** is a possible event that results from a trial. The collection of all possible outcomes for a particular trial is called the **sample space**. For example, when rolling a die, the sample space is the numbers 1 – 6. Rolling a single number, such as 4, would be a single event.

The probability of a single event occurring is the number of outcomes in which that event occurs (called favorable events) divided by the number of total possible outcomes.