A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.

Most brakes commonly use friction between two surfaces pressed together to convert the kinetic energy of the moving object into heat, though other methods of energy conversion may be employed. For example, regenerative braking converts a significant portion of the otherwise wasted kinetic energy of a moving vehicle into electrical energy, which can be stored in batteries for later use. Other methods convert kinetic energy into potential energy in such stored forms as pressurized air or pressurized oil. Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc, fin, or rail, which is converted into heat. Still other braking methods transform the kinetic energy into different forms, for example, by transferring the energy to a rotating flywheel.

Brakes are generally applied to rotating axles or wheels, but may also take other forms, such as the surface of a moving fluid (flaps deployed into water or air). Some vehicles utilize a combination of braking mechanisms, such as drag racing cars equipped with both wheel brakes and a parachute, or airplanes that employ both wheel brakes and flaps raised into the air during landing.

Since kinetic energy increases quadratically with velocity (${\displaystyle K=mv^{2}/2}$), an object moving at 10 m/s has 100 times as much energy as one of the same mass moving at 1 m/s. Consequently, the theoretical braking distance, when braking at the traction limit, is up to 100 times as long. In practice, fast vehicles typically experience significant air drag, and the energy lost to air drag increases rapidly with speed.

Almost all wheeled vehicles have a brake of some sort. Even baggage carts and shopping carts may have them for use on a moving ramp. Most fixed-wing aircraft are fitted with wheel brakes on the undercarriage. Some aircraft also feature air brakes designed to reduce their speed in flight. Notable examples include gliders and some World War II-era aircraft, primarily some fighter aircraft and many dive bombers of the era. These allow the aircraft to maintain a safe speed in a steep descent. The Saab B 17 dive bomber and Vought F4U Corsair fighter used the deployed undercarriage as an air brake.

Friction brakes on automobiles store braking heat in the drum brake or disc brake while braking, then conduct it to the air gradually. When traveling downhill, some vehicles can use their engines to brake.

When the brake pedal of a modern vehicle with hydraulic brakes is pushed against the master cylinder, ultimately a piston pushes the brake pad against the brake disc, which slows the wheel down. In a brake drum design, a similar action is employed, involving a cylinder that pushes the brake shoes against the drum, thereby slowing the rotation.

Brakes may be broadly described as using friction, pumping, or electromagnetics. One brake may use several principles: for example, a pump may pass fluid through an orifice to create friction: