Someone or something that interacts with the system. The Unified Modeling Language (UML) specifies an Actor as a role played by a user or any other system that interacts with the subject.
An Actor is represented by a “stick man” icon with the name of the Actor in the vicinity (usually above or below) the icon.
An Actor models a type of role played by an entity that interacts with the subjects of its associated UseCases (e.g., by exchanging signals and data). Actors may represent roles played by human users, external hardware, or other systems.
An Actor does not necessarily represent a specific physical entity but instead a particular role of some entity that is relevant to the specification of its associated UseCases. Thus, a single physical instance may play the role of several different Actors and, conversely, a given Actor may be played by multiple different instances.
The term “role” is used informally here and does not imply any technical definition of that term found elsewhere in this specification.
When an Actor has an association to a UseCase with a multiplicity that is greater than one at the UseCase end, it means that a given Actor can be involved in multiple UseCases of that type. The specific nature of this multiple involvement depends on the case on hand and is not defined in the UML specification. Thus, an Actor may initiate multiple UseCases in parallel (concurrently) or at different points in time.