1.NounA gear (setting) in a vehicle's transmission which, when selected, makes the vehicle move backward. (Denoted with symbol R on a shifter's labeling.)
"The reverse gear of a Ford Model T was engaged with a foot pedal."
2.NounA gear fitted to a machine which allows it to run in the direction opposite to normal operation.
"This old lathe has a reverse gear, but it wouldn't be used under most conditions, and in fact its use is unsafe under most conditions because the chuck is attached with nothing but a right-hand screw thread!"