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Encyclopedic overview
A UML use case diagram for the interaction of a client (the actor) within a restaurant (the system)
A use case diagram is a graphical depiction of a user's possible interactions with a system. A use case diagram shows various use cases and different types of users the system has and will often be accompanied by other types of diagrams as well. The use cases are represented by either circles or ellipses. The actors are often shown as stick figures.
Excerpted from Wikipedia’s “Use Case Diagram” article, available under the CC BY-SA 4.0 licence.