A shim is a thin, flat piece of material that is used to adjust or maintain a level surface. It can be made from various materials such as plastic, metal, wood, or rubber and is often used in mechanical applications where precise alignment is necessary. Shims are commonly used when assembling machinery or equipment, especially if it requires a specific amount of clearance between two surfaces to prevent friction or wear. They can also be used to level an object or surface that may not be perfectly flat, such as a table or shelf. In essence, a shim is a tool for making small adjustments and maintaining precision in various applications.