A collection of events {A₁, A₂, A₃, … } exhausts the set A if, for the event A to occur, at least one of those sets must also occur; that is, if

S ⊂ A₁ ˅ A₂ ˅ A₃ ˅ …

If the event A is not specified, it is assumed to be the entire outcome space S.