A hubcap is the round metal or plastic cover on the centre of a vehicle wheel. It sits over the hub — the middle part where the wheel meets the axle — to keep dirt out and to make the wheel look finished. Though actually, everyday drivers often call the whole shiny disc a hubcap; a mechanic may keep hubcap for the small centre cap and say wheel cover for the full disc. Cheap ones can pop off on a bump, which is why you sometimes see them in the gutter. Ordinary car talk, not a fancy word.