Definition of driving gear: The gear used to drive its paired gear in a gear pair. An active gear is a gear that inputs motion, while a driven gear is a gear that relies on the drive of the active gear to rotate. The three gears mesh together, and the middle gear is both the driving gear and the driven gear, called the idler gear. Gear transmission relies on the meshing of the driving gear and the driven gear to transmit motion and power. It is currently the most widely used mechanical transmission in machinery.