Architecting and designing : UML 2.5 : Model Elements : Classifiers : Use Case
  
Use Case
A Use Case is a list of actions or event steps typically defining the interactions between an Actor (which is a “role”) and a system to achieve a goal. The Actor can be a human or other external system.
See also
Activity
Actor
Association
DataType
Enumeration
Extension
FunctionBehavior
Information Item
Interaction
Interface
Opaque Behavior
Primitive Type
Signal
State Machine
Stereotype
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Classifiers