An actor is considered an external agent to a system; someone or something that requests a service from the system, or acts as a catalyst for something in the system to occur. UML specifies that the actor is a class of objects, not a particular instance of a class (or object). The actor is defined as a class with a stereotype of actor.

The actor is graphically represented as a stick figure. The actor is defined as a class with stereotype actor. Once drawn on a Use Case diagram, the stereotype actor is automatically applied to the underlying class definition.

To change the actor to a different type of class, select a different stereotype in the drop-down list in the class definition, or type a stereotype not included in the list. The symbol on the Use Case diagram changes from a stick figure to a rectangular class symbol.