reverse gear

noun3

1. •
   A 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. •
   A 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!
3. •
   (figurative) A backward movement; a retreating from a goal; an undoing progress instead of moving forward.