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Locknut
From WikiPedalia
A nut which is tightened against another nut to keep it from loosening up. Often, but not always, there will be a key washer between the locknut and the cone or other part it secures.
Locknuts are used to secure bearing adjustment of most hubs, headsets, pedals and sidepull brakes. Most hubs use locknuts to secure the cones so that they will stay in adjustment. These locknuts are the outermost parts which are screwed onto the axle, and their outer surfaces press against the insides of the fork ends of the frame. The locknuts are therefore the reference point by which axle width and dishing are measured.
