a supplementary nut that is screwed down on a primary nut to prevent it from loosening; a check nut.
A nut used to lock an adjustment nut, or other threaded component, in place. For example, a locknut is employed to keep the adjusting nut on the rocker arm in position.
A nut that is tightened against the primary nut, to keep it from loosening, when the primary nut is tightened to the needed torque. A locknut, also called a jam nut, may be the same as the primary nut or may be made of stamped sheet metal.