If you can give yourself a bath in the tub, it's a bathtub. Like a full bath, covering your entire body with water in ideal conditions. If you're bathing yourself with the tub, like squatting in it as you clean your asshole with a wet rag, or kneeling inside it with the lukewarm water up to your navel, I guess it's just a tub. Bathtubs are more fixed, tubs are more mobile. At least that's how I see it.