A castellated nut, also called a castle nut or slotted nut, is a nut with slots (notches) cut into one end.
The bolt or axle has one or two holes drilled through its threaded end. The nut is torqued properly and then, if the slot isn't aligned with the hole in the fastener, the nut is rotated to the nearest slot. The nut is then secured with a cotter pin or safety wire. Castellated nuts are used in low-torque applications, such as holding a wheel bearing in place.