What is Probability?

An expression of how likely a random event is to occur, as a number from 0 to 1: 0 means the event is impossible, and 1 means it is certain to occur. If the outcomes are equally likely, probability is found as the ratio of the number of favourable outcomes to the number of all possible outcomes. Probability can also be stated as a percentage (0.25 = 25%). It is the foundation of statistics: when drawing conclusions about a whole population from sample data, it is probability that is relied upon.

Source: Britannica: Probability theory