One would think a sketchbook would be full of "sketches" and an art journal would be full of "art." One sounds more humble than the other, and one sounds somewhat pretentious. How do you know the difference? Oh, and by the way, I have never kept a "journal" of any kind in my entire life. I guess I never really felt there was anything about my life worth writing about. So when I think of an art journal, I think of there basically just being some art in it, with the journaling part being entirely optional.
When I look back at the sketches, or "art" I have made, I remember the ideas, feelings, and events that were going on and which inspired me to make the art in the first place. Is that not basically the same as what a journal does, i.e. remind you of the moment? I think rather than "Art Journal, " I might prefer the term "Visual Journal." It seems less highbrow and more true to the way I use it- as a visual way to remember important moments or feelings that have passed.