Use case: A description of a set of sequences of actions, including variants, that a system performs to yield an observable result of value to an
A use case describes an interaction between the user and the system for a specific functionality of the system.

UML includes a diagram type called use case diagrams that can illustrate use cases of a system visually, providing a visual ‘table of contents’ of the use cases of a system.
In the example on the right, note how use cases are shown as ovals and user roles relevant to each use case are shown as stick figures connected to the corresponding ovals.
Use cases capture the functional requirements of a system.