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Probability measures how likely an event or outcome is.
It is usually shown as a success fraction - the number of ways an event could happen compared to the total number of events that could happen - and is written:

probability(of special event) = (No of times that special event occurs) (Total number of events occurring)

A way of measuring the chance that something will happen; if the same experiment is done a very large number of times, the probability of a particular outcome is the proportion of times that it happens.

For a given random experiment and a given event E, if we perform a sequence of trials of the experiment and let r(E) = [number of times E occurs/number of trials in the sequence], then we would expect that as the number of trials in the sequence increases, r(E) will approach a value p(E) which is called the probability of event E.
If E is the whole sample space, then r(E)=1 for every sequence of trials, so p(E)=1.
In general, the probability of an event is equal to the sum of the probabilities of the outcomes corresponding to the event. If the event is E corresponding to n outcomes with probabilities p_i, then p(E) = ∑_i=1 ⁿ p_i