A shim is
anything made out of any material designed to tighten a loose fit.
Washers can be used as shims, steel plates, wood, or anything. Wood shims are used to hang doors and set cabinets. Usually those shims come in the shape of slim
wedges, so they can be moved for a more precise fit. In fitting a door, they can be used in multiples for a tighter fit on wider surfaces. I use shims all the time to secure boxes cut into finished
masonry walls, pounding flat small pieces of electrical
conduit in order to fill the wall.
In the classic book Zen and the Art of Motorcyle Maintenance Robert M. Pirsig cuts a shim from a pop can for his friend's BMW motorcycle. His friend is appalled that such a humble device could be used to perfect such a good example of German engineering. Germans are famous for their precision, Americans for making it work. When the robotically constructed Honda cars arrived in America mechanics were amazed that they needed no shims to fit the doors, unlike the American cars they were so used to. Yet even precise tolerances may relax with use and wear. To Pirsig the engineer who designed the BMW would have appreciated his use of the pop can. His homemade shim is perfect, an essentially free part cut from a soft metal that will serve its intended purpose beautifully.
Softness is a desirable property in a shimming materials. You want a bit of give, and a bit of springiness so it will hold tight. But a shim is made out of anything you have handy. Shimming is an important part of any handyman's bag of tricks.
thanks to dannye for pointing out that Honda's arrival in American revolutionized the tolerances used in american, and European, cars. Today tight fits are the norm, in the old day we pieced things together.