In Domain-Driven Design (DDD), the problem domain refers to the specific area of knowledge or business that the software system is being developed to address. It is the area of expertise that the software is intended to support, such as finance, health care, e-commerce, etc. The problem domain is the context in which the software will be used and the specific requirements that the software needs to meet.

The problem domain is typically defined by the stakeholders of the project, such as the end-users, domain experts, and business analysts. The goal of DDD is to model the problem domain as accurately as possible by creating a domain model that accurately reflects the underlying business concepts and processes. This is achieved by identifying the key abstractions and concepts within the problem domain, and creating a shared understanding of these concepts through the use of a ubiquitous language.

In summary, the problem domain is the specific area of the business that the software system is being developed to address, and DDD is a methodology that helps to model the problem domain accurately and to create a shared understanding of the underlying concepts through the use of a ubiquitous language.