A metal disc with a hole in the middle. There are several different types of washers used on bicycles, to serve different purposes:
Flat washers are commonly used between nuts or bolt heads and the parts that they tighten against. For most fasteners, it is desirable that the part that is turned by the wrench or screwdriver have a washer under it to protect the surface from the turning fastener. Flat washers are also commonly used to spread out the load when a fastener is attached to a thin or fragile part.
Spacer washers are flat washers of various thicknesses just designed to take up space on a threaded shaft.
Star, Split Lockwashers Lock washers have one or more directional teeth that act like pawls against the bearing surface of the fastener. These make it easier to turn the fastener in the tightening direction than in the loosening direction, so it is less likely that the nut or bolt will rattle loose.
Serrated washers have teeth on one side. These are typically used on nutted hubs, with the washer between the axle nut and the forkend. The smooth side faces the nut, and allows the nut to turn without rubbing against the teeth. The serrated (toothy) side bites into the forkend to help keep the axle from slipping forward due to chain tension.
Keyed washers usually have a tab poking into the hole, and fit over a threaded shaft that has a groove ("keyway") for the tab to slide through. Keyed washers are commonly used between a threaded bearing race, either cup or cone, and its locknut. The intent is that when you tighten the locknut, it presses the keyed washer against the threaded bearing race but does not cause the bearing race to turn on its threads, so it stays in adjustment. Keyed washers are almost always used on threaded headsets, often also used on the bearings of pedals.
Formerly, keyed washers were used on hub axles, but that has gone out of favor.
Sturmey-Archer HMW155 Anti-Rotation Washer Anti-rotation washers have a non-round hole and fit on a matching non-round shaft. These washers are designed to keep parts from rotating. Internal gear hubs often have combination serrated/anti-rotation washers. These feature a tab or tabs that fit into the forkend'saxle slot. They are used to keep the hub'saxle from twisting in the frame. (Internal gear hubs have one or more gears mounted on the axle inside the hub, and this gear has to resist some force from pedaling in the higher or lowergears . The force is in opposite directions, depending on the gear selected, so if the axle in not properly secured, it will twist back and forth, causing the axlenuts to loosen up.
Radiused washers are used for mounting caliper brakes to frames/forks. They are flat on one side, the side that the brake caliper is on. They are curved ("radiused") on the opposite side to match the curvature of the brake bridge or fork crown.
