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. Other uses are as a spacer, spring (belleville washer, wave washer), wear pad, preload indicating device, and locking device. Rubber washers are also used in taps (valves) where they form the seal that shuts off the flow of liquid or gas. Usually washers have OD (outer diameter) about twice their ID (inner diameter).

Washers are usually metal or plastic. High quality capscrews require hardened steel washers to prevent the loss of pre-load due to Brinelling after the torque is applied. Rubber or fiber gaskets used in taps to stop the flow of water are sometimes referred to colloquially as washers; while they may look similar, however, washers and gaskets are usually designed for different functions and made differently.

Washers are also important for preventing galvanic corrosion, particularly by insulating steel screws from aluminum surfaces.