Hex nut with two or three, round or rectangular indentations, compressed onto the flat sides of the nut equidistant from each other. The compressions create slightly distorted center threads resulting in a controlled locking action when the threads of the mating part become engaged.
This is the least expensive prevailing torque type of nut, designed for use with machine screws and low-carbon bolts. It allows for automatic assembly because the top and bottom of the nut are identical. It creates a locking action even without being fully threaded onto its mating screw.