From The Collaborative International Dictionary of English v.0.48 [gcide]:
2. A ring of metal, leather, or other material, or a perforated plate, used for various purposes, as around a bolt or screw to form a seat for the head or nut, or around a wagon axle to prevent endwise motion of the hub of the wheel and relieve friction, or in a joint to form a packing, etc.
From WordNet (r) 3.0 (2006) [wn]:
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