Brake booster

[1] A mechanical device which attaches to the brake system to multiply the force the driver applies with his foot (or hand as in the case of a motorcycle). The device uses air, vacuum, or hydraulic fluid to accomplish this purpose. Sometimes called "power assisted brakes", "vacuum assisted brakes", "hydraulically assisted brakes", or just " power brakes". In most cars, the boost comes from engine intake vacuum. In motorcycles it comes from hydraulic fluid.
[2] An arch (usually built from aluminum or carbon fiber) which attaches to the brake bosses of a bicycle in conjunction with the brake. The powerful braking force of V-brakes or linear pull brakes can cause frames and forks to flex and waste energy which could have been applied to the rims. The booster acts as a brace to prevent frame flex induced during braking.
See also hydraulic brake booster, vacuum brake booster