Domain

A Domain is the top-level area of activity that the model describes. It is the answer to the question what business are we in? – not a single feature, not a product, but the whole field within which the system operates.

A model has exactly one Domain. It sits above Subdomains and everything below them, and serves as the natural boundary: when you ask "is this in scope?", the answer is "yes if it falls under this Domain".

When to Use the Term

Use Domain when you want to talk about the whole. A logistics company has a logistics Domain. A bank has a banking Domain. A municipal library has a library Domain. The Domain is the umbrella under which all the other terms make sense.

Resist the temptation to split a Domain into more than one. If you find yourself wanting to, what you're really discovering is that you have multiple Subdomains, not multiple Domains.