Probability

Probability measures the long-run relative frequency of an event over many repetitions. For equally likely outcomes, $P(A) = \frac{\text{number of outcomes in } A}{\text{total number of outcomes}}$. For example, rolling an even number on a fair six-sided die has probability $3/6 = 0.5$. Over many rolls the fraction of even results settles near this value, an idea called the law of large numbers.