A use case is a description of how a system’s behavior in response to a request from a stakeholder known as an actor. The actor could be a person or an external system that interacts with the system being described.
The actor initiates an action with the system with the purpose of accomplishing a goal. The system responds to the actor’s action in a way that fulfills the interests of all of its stakeholders. A use case summarizes a complete series of related scenarios that may unfold.