PeterD wrote:
"unproved theorem"

(This term ranks up there with "half pregnant".)

I think it might depend upon the context. I know some books present a theorem and then present the proof. They also have a few theorems that are true, and have been proven, but they might require more advanced mathematics than the student has achieved. But they need to use the theorem for other proofs, and so it is presented, and stated to be true, but is unproved (from the student's perspective).

But there's another possibility, too. Technically, isn't a theorem something that is either true or, if unproved, something everyone believes to be true? Fermat's last theorem was one of these when I was in school (though I believe it has since been proven). Also, I don't think there is a proof of Euclid's theorem that given a line and a point not on the line that there is one and only one line through the point that is parallel to the line. Even if unproved this statement would be believed to be true by most people.

Some of this may really just be language and not mathematics, for there are surely unproven statements that are assumed to be true, whether you want to classify these as "theorems" or not seems to be more a question for the English classroom than the mathematics one.

EDIT: ThomasGR beat me on the language thing.