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It is theory of chance when taken as science.
It is chance of happening an event when considered in connection with the event. Probability of any event is between 0 and 1, both included.
Mathematical or Objective Probability
Probability theory, which is based on statistical data and probability axioms, is called as mathematical probability.
Axioms of Probability.
There are three axioms of probability: (1) Chances are always at least zero. (2) The maximum chance that something happens is 100%..
Probability theory, which is based on feeling or thinking of an person, is called as subjective probability.
It is probability of an event that is calculated on the assumption that some related has happened.
Action whose outcomes are of interest to us is called as an experiment. e.g. toss of a coin.