A nyloc nut, also referred to as a nylon-insert lock nut, polymer-insert lock nut, or elastic stop nut, is a kind of locknut with a nylon collar insert that resists turning.

The plastic insert is placed at the end of the nut, with an inner diameter (ID) slightly smaller than the major diameter of the screw. The insert deforms elastically over the threads of the screw, without having threads in turn cut into it. The nylon insert locks the nut by friction against the screw under the applied radial compressive force.