| A locking differential is a drivetrain component that can at the operator's option be set so as to disallow any speed difference between the shafts that it powers. This is useful mostly in off-road driving, where individual wheels may bog down but you want them to continue turning. It does bad things to the vehicle's cornering performance, however, as the inner wheel can no longer turn slightly slower than the outer one.
The most well-known manufacturer of these devices for use in the U.S. is Quaife, a British company. | 
