[t]he extent to which a certain event is likely to occur, measured by the ratio of the number of times that the event does occur to the total number of cases possible.^{[1]}

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[a] numerical value between zero and one assigned to a random event (which is a subset of the sample space) in such a way that the assigned number obeys three axioms: (1) the probability of the random event "A" must be equal to, or lie between, zero and one; (2) the probability that the outcome is within the sample space must equal one; and (3) the probability that the random event "A" or "B" occurs must equal the probability of the random event "A" plus the probability of the random event "B" for any two mutually exclusive events.^{[2]}

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[the] likelihood that is expressed as a number between zero and one, where zero indicates that the occurrence is impossible and one indicates definite knowledge that the occurrence has happened or will happen, where the ratios between numbers reflect and maintain quantitative relationships.^{[3]}

Probability is generally expressed as a number between 0 and 1, where 0 indicates that the occurrence is impossible and 1 indicates definite knowledge that the occurrence has happened or will happen.