1. Organized around a main idea or theme
2. Created to illustrate a concept

Idea; in philosophy, the term “concept” has superseded the more ambiguous “idea”. To have a concept of dog is to be able to distinguish dogs from other things, or to be able to think or reason about dogs in some way.
Conceptual realists hold that concepts are objectively existing universals, like real essences. Conceptualists hold that universals are mind-dependent concepts (this is the outlook of nominalism).