The probability of an event is the likelihood of that event occuring. Probability is a value between (and including) zero and one.

If P(E) represents the probability of an event E, then 0 ≤ P(E) ≤ 1.

An experimental probability is measured.

A theoretical probability is calculated.

An event with a probability of zero [P(E) = 0] will never occur (an impossible event).

An event with a probability of one [P(E) = 1] means the event must occur (a certain event).

An event with a probability of 0.5 [P(E) = 0.5] is sometimes called a fifty-fifty chance event or an even chance event.

An event with a higher probability is more likely to occur than one with a lower probability.