Note that on a Construction diagram, an Actor element defaults to Rectangular Notation and looks like a Class element.
An Actor is a user of the system; user can mean a human user, a machine, or even another system or subsystem in the model. Anything that interacts with the system from the outside or system boundary is termed an Actor. Actors are typically associated with Use Cases.
Actors can use the system through a graphical user interface, through a batch interface or through some other media. An Actor's interaction with a Use Case is documented in a Use Case scenario, which details the functions a system must provide to satisfy the user requirements.
Actors also represent the role of a user in Sequence diagrams, where you can display them using rectangle notation. Enterprise Architect supports a stereotyped Actor element for business modeling. The business modeling elements also represent Actors as stereotyped Objects.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, pp.640/647) states:
An Actor specifies a role played by a user or any other system that interacts with the subject
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.
NOTE. 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.
NOTE. 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 this specification. Thus, an Actor may initiate multiple UseCases in parallel (concurrently) or at different points in time.