Definitions for "Use Case Diagram"
Keywords:  uml, diagram, actor, case, graph
UML notation used during the development and analysis of requirements to represent the functionality of the system use case describes a function of the system in terms of a sequence of interactions between an actor and the system. A use case also includes entry conditions that need to be true before executing the use case and the exit conditions that are true at the completion of the use case.
a diagram that documents which use cases are used by which externals and any relationships between the use cases.
A UML diagram showing the relationships between Actors and Use Cases. See Section 3.3, "Output of the Requirements Capture Process" for more information.