Metal Self Locking Nuts - Philidas Nuts
Self locking nuts work by using increased friction between the threads of the two mating faces to cause a slight distortion of the threads. They are frequently used in areas where vibration is experienced.
Self locking nuts can be re-used, but only a few times before they lose their inherent locking qualities.
Stiff nuts, jam nuts and bent beam nuts are all terms that are used to describe self-locking nuts.