Probability is a concept that numerically measures a degree of uncertainty and therefore of certainty of the occurrence of events.
An action that results in one of several outcomes is called an experiment.
An experiment is called random if it has more than one possible outcome and cannot be predicted or determined in advance. i.e. Tossing a coin, Rolling a dice.
The set of possible outcomes or the totality of all possible outcomes of an experiment constitutes the sample space.