A castellated nut, also called a castle nut or slotted nut, is a nut

Nut (hardware)

A nut is a type of hardware fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads' friction, a slight stretch of the bolt, and compression of the parts...

 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

Screw

A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...

 or axle has one or two holes drilled through its threaded end. The nut is torque

Torque

Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

d 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

Cotter pin

A split pin, also known in U.S. usage as a cotter pin or cotter key, is a metal fastener with two tines that are bent during installation, similar to a staple or rivet. Typically made of thick wire with a half-circular cross section, split pins come in multiple sizes and types.The British...

 or safety wire

Safety wire

Safety wire or lockwire is common in the aircraft and racing industries as an extra precaution to keep vital fasteners from unintentionally loosening and parts from falling off due to vibration or other forces. The use of safety wire is a type of positive locking device. It also allows rapid and...

.

Castellated nuts are used in low-torque applications, such as holding a wheel bearing

Rolling-element bearing

A rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing round elements between the two pieces...

in place.