The original point was it's not-a-prop. In my experience, what they built would be considered a set (Paradise City in ST:V was a set) or a set piece. I said there are gray areas.
Paradise City was a set because it was a whole environment in which the action took place
. That is the definition of the word "set." It's the entire "stage," not just a piece of the stage. The obelisk was not an environment, it was one object within an environment. The "set" was the entire lakeside. The obelisk was a set piece.