's double cheeseburger. Though it sounds like it should be a quadruple, the menu clearly explains that this means double meat and double cheese.
I've heard but never tested that they'll gladly make you a 3x3 or 4x4 (even stranger names, but you get the idea) if you ask for it. In fact, there's a whole world of unlisted items available for purchase that just aren't listed on the menu. The only ones I can remember right now are animal style, wishburger, and grilled cheese.