Sometimes written as lock nuts, they also go by various other names, including half nuts, jam nuts and thin nuts.
Locknuts are used to lock a full nut in position to prevent it from working loose as a result of vibration or other factors. A common area in which lock nuts are used is on car wheels.
Locknuts are often mixed up with Nyloc nuts, which are different. Nyloc nuts are self-locking nuts that feature a nylon collar in the top portion of the nut that locks the nut in place.