9/9
I also missed the last question.
I still don't get why Octavarium isn't considered a concept album. Maybe it doesn't tell a story, but there are recurring musical elements between songs, the end mirrors the start, the ascending keys in the songs, the many references to 5 and 8, etc.
The bolded part answers your question. The term "concept album" does not simply mean an album with a concept. It means there is generally a story with characters that plays out over the course of the album. Octavarium, and perhaps Six Degrees, are more "theme albums." Does it make a whole lot of difference? Not really. But again, to answer your question, the term "concept album" generally has a specific meaning, which is what the question was getting at.
OK. It's just, I always thought "concept album" was a broader spectrum. When I was a teen and started reading magazines/books/webs about prog and other rock sub-genres,
Dark Side Of The Moon or
Tales From Topographic Oceans used to come up as examples of concept albums, and neither has a plot/characters. So, I considered that, if an album features recurring musical elements or the lyrics/music focuses on a subject (and both examples do both things), then it can be considered conceptual.
But, I get your point. To me both terms mean basically the same thing, though, since I mostly pay attention to the music.