Recall that we defined the probability of event A as
This is called the classical probability definition. Another way to interpret probability is as the long-run relative frequency (long-run fraction) of the event. That is, if I flip a fair coin hundreds and hundreds of times, the fraction of heads will be very close to 0.5. The more I repeat the experiment, the closer to 0.5 the relative frequency will be. This is the same result the classical definition gives us. The relative frequency interpretation of probability works especially well for repeatable events, e.g., flipping a coin, rolling dice, drawing cards, etc. See the ``Relative frequency interpretation of probability'' concept lab for an example.