Washer is machine component that is used in conjunction with a screw fastener such as a bolt and nut and that usually serves either to keep the screw from loosening or to distribute the load from the nut or bolt head over a larger area. To prevent loosening, several other types of washers are used.

Threaded fasteners stress the material in which they are driven. Driving a screw into wood, for example, may cause the wood to crack around the surface. Washers reduce the risk of such damage by evenly distributing the fastener's load across the surface of the material. Not all materials require the use of washers.

As the bolt is further tightened, the washer exerts an equal and opposite force until the nut no longer turns. Used in conjunction with a regular flat washer, the bolts and nuts tighten without laying waste to the surface or the fastener hole. A flat washer and alloy fastener work together to balance applied torque.