A shim is a thin and often tapered or wedged piece of material used to fill small gaps or spaces between objects). Shims are typically used to support, adjust for better fit, or provide a level surface. They may also be used as spacers to fill gaps between parts subject to wear). Shims can be made from many materials, including wood, stone, plastic, metal, or even paper, depending on the context). High-quality shim stock can be bought commercially, for example, as laminated shims, but shims are often created ad hoc from whatever material is immediately available).

Shims have various applications in different fields. In automobiles, shims are commonly used to adjust the clearance or space between two parts, such as bucket tappets to control valve clearances). In assembly and weld fixtures, precision metal shims are used between two parts so that the final production parts are created within the product drawings specified tolerances. On machinery installations such as pumps and motors, shims are used under every equipment support foot to guarantee flexibility for adjustments, like a slight raising or lowering of a motor, when parts of the machinery).

In construction, shims are used to make slight adjustments to home elements, such as door frames and windows, to make them plumb (perfectly vertical) and level. Wooden shims are a great way to bridge the gaps when installing trim, doors, windows, or cabinetry projects. They also help to level floors, tables, or chairs. Plastic shims work great for toilets, bathrooms, subfloors, or exterior elements.

In computer programming, a shim is a small library that acts as a middleman and partially or completely changes the behavior or operation of a program. It intercepts an API, changes the parameters passed, handles the operation itself, or redirects the operation elsewhere. Shims can also be used for running programs on different software platforms than they were developed for.