Domain events has nothing to do with system events, event sourcing, any technical patterns or architectural pattern like CQRS.
Domain events are something that actually happens in your customers business, not in your software.
Below is an excerpt（摘录，引用） from the domain event pattern, as defined by Eric Evans.
Model information about activity in the domain as a series of discrete（不相关联的，非连续的） events.
Represent each event as a domain object. These are distinct from system events that reflect activity within the software itself, although often a system event is associated with a domain event, either as part of a response to the domain event or as a way of carrying information about the domain event into the system.
A domain event is a full-fledged（羽翼丰满的） part of the domain model, a representation of something that happened in the domain.
Ignore irrelevant（不相干的，不恰当的） domain activity while making explicit the events that the domain experts want to track or be notified of, or which are associated with state change in the other model objects. – Eric Evans