What is a domain event (an event in the domain)?

A Domain Event is a fact that happened in the domain that has business meaning and can trigger other actions.

Characteristics:

1. Past tense: the event has already happened (OrderPlaced, PaymentFailed).
2. Independence: the event describes a business fact, not a technical action.
3. Carrying context: other parts of the system can react to it without knowing the details of its source.

Example: After OrderPaid, delivery can be triggered automatically, or bonuses can be credited.

The point: domain events make it possible to build loosely coupled, reactive modules where logic is extended without changing the core.