The cap that fits over the outside of a wheel on some vehicles to keep dust and water away from the wheel bearings and brakes. Considered a decorative accessory on many cars, it's also a safety device that causes quite a racket if a lug nut falls off and tumbles around inside it.
A hubcap, wheel cover or wheel trim is a decorative disk on an automobile wheel that covers at least a central portion of the wheel. Cars with stamped steel wheels often use a full wheel cover that conceals the entire wheel. Cars with alloy wheels or styled steel wheels generally use smaller hubcaps, sometimes called center caps.