A partition of an event A is a collection of events {A₁, A₂, A₃, … } such that the events in the collection are disjoint, and their union is A. That is,

AjAk = {} unless j = k, and

A = A₁ ˅ A₂ ˅ A₃ ˅ … .

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