brake block

Short definition from Wiktionary

Noun

1. 1
   One of two small blocks fitted to a brake caliper, which rub on the wheel rim when the brake is applied.
2. 2
   A block fitted to locomotives or rolling stock, which rubs on the steel tyre of the wheel when the brake is applied.
3. 3
   A block (perhaps wooden) used for braking a horse-drawn vehicle.