An actor is a concept used to represent an external entity or system that interacts with the system being modeled. An actor can be a person, organization, or any other system that provides inputs to or receives outputs from the system being modeled.
Actors are used to define the stakeholders, requirements, and interfaces of a system. They are typically represented graphically in use case diagrams and can also be described in more detail in other model elements such as blocks and internal block diagrams. By representing actors in SysML, engineers can gain a better understanding of the interactions and relationships between the system being modeled and its external environment.