...or
any thing, the way some folks toss around the term
This really gets down to a semantic issue: "Progressive rock" as a phrase means (at least) one thing as a logical understanding of the definitions of those two words when combined. But it -- and its shortened form "prog-rock" -- has come to mean
something else entirely, because it now represents a
genre of music that
may have actually been "progressive" at one time, but now if it is, it only is due to coincidence. "Prog-Rock" the genre is
not by definition rock that is progressive; now it's just music that conforms to the tropes that grew out of the original bands that were considered "progressive" ~40-odd years ago.