air brake

  1. a brake operated by the action of compressed air on a piston
  2. popularly speed brake

See air brake in American Heritage Dictionary 4

noun
  1. A brake, especially on a motor vehicle, that is operated by compressed air.
  2. A surface, such as an aileron, that can be projected into the airflow in order to decrease the air speed of an airplane by increasing drag; a spoiler.

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