A washer is a thin disk with a hole, usually in the middle. It is normally used to support the load of a threaded fastener. Other uses are as a spacer, spring (belleville, wave washers), 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.

Washers are usually metal or plastic. High quality capscrews require hardened steel washers to prevent the loss of pre-load after the torque is applied (due to Brinelling). 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 made differently and designed for different functions.

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