Events that do not have any common elements and for which their assembly corresponds to set of all possible of a random experiment.
The expression complementary events is a synonym for contrary events.
Notation
The complementary event of an event E is noted as E’.
Therefore, we have E ∪ E’ = Ω.
Example
In the random experiment that consists of rolling a fair die with six faces numbered 1 to 6 and recording the result that appears on the top face, the events “A: getting an even result” and “A’: getting an odd result” are complementary events, because their combination corresponds to the set of possible outcomes, which is: Ω = {1, 2, 3, 4, 5, 6}.