An event is a subset of the sample space related to a random experiement. It is described by a sentence or by its explicite set.

Example: if we have the sample space of Math and Physics books S = {Math1, Math2, Math3, Phys1, Phys2} , the event : "Get a Physics book" described by a sentence can be described by a its related set {Phys1, Phys2}

A event is certain if it is always realized (example: get a scientific book from S). It is impossible if it can not occur at all (example get an history book from S). It is denoted by an empty space Φ. An event is simple if it contains only one result from S (one element); otherwise it is a compound event.

Operations on the events are the operations on the sets. The event conjunction of two events A and B denoted by A∩B is the event that occur if both events A and B occur together. The event disjunction of two events A and B denoted by A∪B is the event that occur if the events A or B occur. The contrary event of an event A is denoted by A' and it is realized if A is not.

Two events are mutually exlusive or incompatibles if they cannot be realized simultaneously, that is their conjonction is an impossible event. Two events are independents if the probability of each does not affect the brobability of the other.