Washer, 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. For load distribution, thin flat rings of soft steel are usual.
Threaded fasteners do a more reliable job when accompanied by washers. Most notably, washers protect the surface from damage during installation. They distribute the pressure and prevent the fastener from moving or corroding. Skipping on washers can dramatically reduce the lifespan of how your product is put together.