Largely a sequence of interactions across the system boundary between the actor(s) and the system.

Natural language is used so that non-technical as well as technical users can understand it. The use case is a vehicle for getting functional requirements out of users. It gives them hooks to hang ideas on and is a very powerful interview tool, especially when used in conjunction with screen prototyping.

A good use case is also:

	Easy to understand
	Unambiguous
	Complete including all alternate flows
	Specified in a document separate from the use case diagram
	Agreed with the users of the system
	Mapped to a prototype screen or system interface prototype