a round, flat disc made of steel or cast iron. It is mounted on outside of wheel hub.

A brake disc is a circular plate against which a brake lining is forced by hydraulic pressure.  The purpose of this is similar to the purposes of the brake assembly described under the brake cylinder. [..]

Each wheel of a car has its own brake. The most common type of brake used on modern cars is the disc brake. This is a rotating metal disc that is fixed so that it always turns with the wheel. When the brakes are applied, a clamp (called a brake calliper) squeezes the disc, slowing the rotation of the wheel.