49 C.F.R. § 570.54
Unless otherwise indicated, all terms used in this part that are defined in part 571 of this chapter, Motor Vehicle Safety Standards, are used as defined in that part.
Air-over-hydraulic brake subsystem means a subsystem of the air brake that uses compressed air to transmit a force from the driver control to a hydraulic brake system to actuate the service brakes.
Electric brake system means a system that uses electric current to actuate the service brake.
Vacuum brake system means a system that uses a vacuum and atmospheric pressure for transmitting a force from the driver control to the service brake, but does not include a system that uses vacuum only to assist the driver in applying muscular force to hydraulic or mechanical components.
49 C.F.R. § 570.54