Referencing [Evans], you will find no formal definition for Domain Events. The pattern was introduced in detail sometime after the book was published. To begin a discussion about implementing Events in the Domain (2), consider the contemporary definition:
Something happened that domain experts care about.
Model information about activity in the domain as a series of discrete events. Represent each event as a domain object. . . . A domain event is a full-fledged part of the domain model, a representation of something that happened in the domain. [Evans, Ref, p. 20]
How can we determine if something that happens in the domain is important to the domain experts? As we have discussions with them, we must listen ...