Refers to the act of entering a state during the execution of a state machine. The state machine keeps track of which state is active and its behaviour is defined based on the active state(s). A state may declare entry actions, which will be executed when a state is entered.

If a state is a compound state, the defined initial state is also entered, since in an active compound state, exactly one substate must also be active.

If a state is a [parallel state], each region and their initial states will also be entered.

Entry of a state is defined to be instantaneous for all practical purposes; this includes the execution of any entry actions, and the entry of all substates. In other words, a state can never be “half entered” or “half active”.