Event

An event is a measurable subset of a probability space.

The event that $A$ does not occur has probability $1-P(A)$. Conditioning on an event positively associated with $A$ negatively associates its complement with $A$.

Two events are disjoint when they cannot both occur. For a finite family of pairwise disjoint events, the probability of their union is the sum of their probabilities, so that sum is at most one. This condition differs from the condition defining independent events.