The likelihood or chance of a given event happening. It is often expressed as a fraction or decimal. The probability that m particular events will occur out of a total of n possible events is m/n. A certainty means that, out of n possible events, all the events (n events) will happen. Therefore, a certainty has a probability of 1 (n/n = 1). Similarly, an impossibility has a probability of 0 because none will happen out of the total n possible events (0/n = 0).

For example, the chance of getting heads is the same as the chance of getting tails when tossing a coin. That is, it is reasonable to expect heads once and tails once for every two tosses. Hence, the probability of getting heads (or tails) is 1/2 or 0.5.

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