An event^[1] in the Unified Modeling Language (UML) is "something that may occur at a specific instant in time".^[1]

Events can, but do not necessarily, cause state transitions from one state to another in state machines^[2] represented by state machine diagrams.

Consider "the case of two Transitions^[3] originating from the same State,^[2] triggered by the same event, but with different guards. If that event occurs and both guard conditions are true, then at most one of those Transitions can fire in a given run-to-completion step."^[3]