An actor is an external person, organization, system, device, or process that interacts with a system.
Actors can start a use case, exchange information, or receive a result. Therefore, they help define the system boundary and its interfaces.
In a use case diagram, actors appear outside the system boundary. Association lines connect them to relevant use cases. For example, a customer can start a login process. Likewise, a scheduler can trigger a timed process.
Every actor represents a stakeholder, but not every stakeholder acts as an actor. For example, a sponsor may influence requirements without interacting with the system.
Actors help teams identify users, external systems, triggers, and required system functions. As a result, they support clear scope definition and requirements analysis.
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