When is it metal?

Yeah when does it stop being rock and become metal? Is it a certain way they have to play the drums/guitars? I was thinking the speed but look at doom metal.

Feel. In the end it comes down to what the band's usually expressing, and the kind of sound they're going for. Intention, if that means anything. Like, I'd say Saxon was still in the region of Hard Rock, simply because their songs rock. As opposed to Judas Priest, who may sound similar but the intention is somewhat different.