It is an area known and understood by a Domain Expert. The Problem Domain represents the field of endeavor to be implemented into working code. The problem domain includes domain concepts. What does it not include? Concepts and functionality related to human interaction (UI), system integration, data management and infrastructure.
Domain. [right of ownership, dominion (Greek)] The sphere or field of activity or influence; as the domain of art or politics
Problem. [to throw forward, to drive forward (Greek)] A question proposed for solution or consideration [Webster’s, 1977]
A Problem Domain is therefore, “a field of endeavor under consideration”.
Domain, Business Domain
In common usage, the terms Domain and Problem Domain are often imprecise and used interchangeably. One will commonly see confusion and differing use the terms. This problem of unclear boundaries is often driven by inconsistent interpretations of what makes up a Layered Architecture.