An encapsulation of the behaviour associated to one state in an FSM. This behaviour consists of: actions that are associated to the FSM state. an entry check that verifies whether the state can be entered. an exit check that verifies whether the state can be exited. a termination check that verifies if all actions associated to the current state have been executed. a next state that must be entered when the actions associated to the current state have been completed. The actions that are associated to a state are of three types: An initialization action: a punctual action performed immediately after the state is entered A continued action: an action performed continuously while the state is active An exit action: a punctual action that is performed immediately before the state is exited Finally, to each state, a next state may be associated. If B is the next state associated to state A, then the FSM will perform an autonomous transition to state B after execution of all the actions associated to state A has been completed (i.e. after its termination check reports "terminated").
See the Finite State Machine design pattern.