A split 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...

 that consists of two pieces which can be clamped together to engage the thread of the nut with the bolt, or released, to allow the nut to move along the bolt without turning the bolt. The two pieces have chamfered ends (60° to axis). The nut is split lengthwise so that it may be opened for quick adjustment. A split nut assembly is often used in positioning systems, for example in the lead screw of a lathe

Lathe

A lathe is a machine tool which rotates the workpiece on its axis to perform various operations such as cutting, sanding, knurling, drilling, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation.Lathes are used in woodturning,...

. This type of nut can be used only for trapezoidal threads.