Random experiment: If an experiment, when repeated under identical conditions, do not produce the same outcome every time but the outcome in a trial is one of the several possible outcomes than such an experiment is known as a random experiment.

Sample Space: A sample space, denoted by S, associated with a random experiment is a set of points such that (i) each element of S denotes an outcome of the experiment and (ii) any performance of the experiment results in an outcome that corresponds to exactly one element of S.

The element of the sample space associated with a random experiment are called the elementary or simple events of that experiment.

Event: An event (associated with a random experiment) is a subset of the sample space S (associated with that experiment).

Mutually exclusive events: A set of events is said to be mutually exclusive if the occurrence of one of them precludes the occurrence of any of the remaining events.

Equally likely events: A set of events is said to be equally likely if, taking in to consideration all the relevant factors, there is no reason to expect one of them in preference to the others.

Exhaustive Events: A set of events is said to be exhaustive if the performance of the experiment always results in the occurrence of at least one of them.