In Domain-Driven Design (DDD), a domain expert is a person who has deep knowledge and understanding of the business domain or problem area being addressed by the software system. The domain expert has expertise in the business rules, processes, and concepts relevant to the system being built and serves as a primary source of information for the development team. The domain expert helps ensure that the domain model accurately represents the business domain and that the system being built solves the right problems and addresses the right business needs.