The complement of an event is the "opposite" of that event. That is, if the event says that something will occur then the complement of the event is that the thing will not occur. If the event says something is true, the complement of the event is that it is not true.
- Event: it will rain today; complement of event: it will not rain today.
- Event: the triangle ABC contains a right angle; complement of event: the triangle ABC does not contain a right angle.
- Event: a flipped coin lands "heads"; complement of event: the coin lands "tails"
If an event is denoted by , then the complement of the event is represented by any of the notations , , or .