In Domain-Driven Design (DDD), a domain object refers to an object that represents a concept or an element in the problem domain. It is a fundamental building block of the domain model, and is used to model the business entities, value objects, and services that make up the domain.

A domain object can be either an entity, which is a domain object with a unique identity, or a value object, which is an object that represents a characteristic of the domain without having an identity of its own.

Domain objects are the central focus of DDD, as they are used to implement the business logic and state of the domain, and provide a representation of the business concepts and objects in the problem domain. They are used to model the business rules, constraints, and relationships that define the behavior of the business, and provide a way to encapsulate the complex business logic in a clear and concise manner.