In Event Storming, an Actor represents any external entity (human, system, or service) that interacts with the system you are modeling. Actors initiate events by performing actions or triggering processes within the system. They play a crucial role in shaping the flow of events, but they are always external to the core business process you are analyzing.

Key Aspects of Actors in Event Storming:

1. External to the System: Actors are not part of the internal system or process but are external entities that interact with it.

2. Triggers Events: Actors initiate domain events by performing certain actions, like placing an order, submitting a form, or making a request.

3. Human or Non-human: Actors can be people (like customers or employees) or external systems (such as third-party services, APIs, or machines).

Example:

Imagine you're modeling an e-commerce system using Event Storming. Some possible actors could include:

- Customer: A person who browses the website and places an order.

- Payment Gateway: An external system that processes payments.

- Warehouse System: A system that manages inventory and shipping.

In the Event Storming process, these Actors would be noted as key entities that trigger specific events like "Order Placed" or "Payment Processed."

In summary, the Actor is the source of domain events in Event Storming, making them a critical part of understanding the external interaction points with the system.