A washer is a thin plate (typicallydisk-shaped) with a hole (typically in the middle) that is normally used todistribute the load of a threaded fastener, such as a screw or nut. Other usesare as a spacer, spring (bellevillewasher, wave washer), wear pad, preload indicating device, locking device, andto reduce vibration (rubber washer). Washers usually have an outer diameter(OD) about twice larger than their inner diameter (ID).

Washers are usually metal or plastic.High-quality bolted joints require hardened steel washers to prevent the lossof pre-load due to Brinelling after the torque is applied.

Rubber or fiber gaskets used in taps (orfaucets, or valves) to stop the flow of water are sometimes referred tocolloquially as washers; but, while they may look similar, washers and gasketsare usually designed for different functions and made differently.

Washers are also important for preventinggalvanic corrosion, particularly by insulating steel screws from aluminiumsurfaces.