Use Case Definition
A use case is the specification of a set of actions performed by a system, which yields an observable result that is typically of value for one or more actors or other stakeholders of the system.
Actors are the entities that interact with a system. Although in most cases, actors are used to represent the users of system, actors can actually be anything that needs to exchange information with the system. So, an actor may be people, computer hardware, other systems, etc.
The scope of a system can be represented by a system (rectangle shape), sometimes referred to as a system boundary. The use cases of the system are placed inside the system shape, while the actors interacting with the system are placed outside the system. The use cases in the system constitute the total requirements of the system.
References
Use Case Diagram – A Notation Guide
