differential gear

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An arrangement of gears in an epicyclic train permitting the rotation of two shafts at different speeds, used on the rear axle of automotive vehicles to allow different rates of wheel rotation on curves.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An arrangement of bevel gears permitting the rotation of two shafts at two different speeds whilst supplying the same torque; used on the rear axles of automobiles to aid cornering.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a bevel gear that permits rotation of two shafts at different speeds; used on the rear axle of automobiles to allow wheels to rotate at different speeds on curves