A strong U-shaped bar fastened to the frame of a vehicle, such as an off-road vehicle or a race car to protect the driver and passengers from being crushed in the event that the vehicle rolls over.
2002, Bobby Alvarez, Ghost Runner on Second[1], page 163:
Once the two left wheels departed the surface, Travis knew he was in for trouble. Since there wasn't a roll bar on the SUV, the roof was probably going to collapse on his head when it contacted the asphalt.