A washer is a thin plate (typically disk-shaped) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a screw or nut.

Increase the bearing surface of a nut or fastener's head, provide a smooth mating surface or protect the surface from damage when the nut or head is rotated, compensate for oversized or irregular shaped holes, space or shim parts, or to resist loosening.

Other uses are as a spacer, spring (belleville washer, wave washer), wear pad, preload indicating device, locking device, and to reduce vibration (rubber washer). Washers usually have an outer diameter (OD) about twice the length of their inner diameter (ID).

Depending on the washer type, common materials include aluminum, brass, nylon, silicon bronze, stainless steel and steel. Finishes include nickel for brass and steel, and hot dip galvanizing and zinc platied(blue,yellow,white),black,for steel;polish for stainless steel.