Nylock nuts or often referred to as nylon-insert lock nut, polymer-insert lock nut, or elastic stop nut. The nylon collar increases the friction in the screw thread.
Nylon insert lock nuts are a common torque, locking nut that resists loosening caused by vibration and everyday use. Lock nuts, as opposed to free-spinning hex nuts, use a deforming elastic or metal material to maintain their position in the face of torque and stress.