The Collaborative International Dictionary of English v.0.48:
Lock washer \Lock" wash`er\, n. A washer that is not completely smooth or flat, designed to prevent a nut from loosening, placed under the nut on a screw or bolt. [1913 Webster]WordNet (r) 3.0 (2006):
lock washer n 1: washer that prevents a nut from loosening [syn: lockring, lock ring, lock washer]