Artifact: Use Case
A Use Case defines a sequence of interactions between an actor (a role played by a human, system, or device) and an application. The actor initiates the interaction in order to achieve a goal. Each ellipse in the Artifact: Use Case Model is a Use Case.
More detail: Purpose - Audience - Template
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Purpose
A Use Case defines functional requirements for an application by describing what the application does in response to an actor's actions. The Role: Requirements Analyst writes each Use Case using the same terms used by the Role: Subject Matter Experts so the experts and Role: Stakeholders can review and agree to the functional requirements.
Good Use Cases are written with usage in mind - what the users (human actors) are trying to achieve. Good Use Cases do not describe the user interface design; they describe the underlying functional requirements.