Jack definition1. A device for lifting the vehicle, or part of the vehicle, off the ground to facilitate repairs. The most common jacks are the bumper , tripod , scissors , and hydraulic jacks .
2. A characteristic of swing-axle rear suspensions . Cornering forces can act on these suspensions to lift the body of the vehicle so that the outer wheel tucks or jacks under the vehicle. When carried to the extreme, jacking forces could tip the vehicle over.