Shim

A shim is a thin strip or tapered wedge of material inserted to fill a small gap, level an uneven surface, or fine-tune an alignment -- installing a door or cabinet plumb and square against a wall that isn't quite either, or fine-tuning a table saw's fence or blade alignment by a hair. Wood shims (often simple cedar shingle stock, thin and naturally tapered) are the traditional material, though plastic and composite shims are common in construction and cabinetry installs where consistent, uniform thickness matters more than the taper a wood shim provides.