What do you call objects returned by an API, and what "layer" do they reside on?

When building a class library with a public API, I often have a static interface with methods or properties that return objects, and sometimes the methods will even take objects as arguments. What is a good term for objects of this type? I'd like to have a properly-named namespace where these classes reside.

The API layer is usually just as single static class interface which sits on top of a business logic / domain layer. Should the public API classes reside in the domain logic layer alongside potentially other, internal classes used in that layer?