Wikipedia
Castellated nut
A castellated nut, also called a castle nut or slotted nut, is a nut with slots (notches) cut into one end. The name comes from the nut’s resemblance to the crenellated parapet of a medieval castle.
The bolt or axle has one or two holes drilled through its threaded end. The nut is torqued properly and then, if the slot is not aligned with the hole in the fastener, the nut is rotated forward to the nearest slot. The nut is then secured with a split pin, R-clip or safety wire. It is a positive locking device.
Castellated nuts are used in low-torque applications, such as holding a wheel bearing in place.