M5 Hexagon Nylon Insert Nut

Hexagon Nuts with Nylon Inserts - A2 Stainless Steel

A nylon insert nut, also referred to as a nyloc 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 slightly smaller than the major diameter of the screw.