Shim

Shim - 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 in order to support, adjust for better fit, or provide a level surface. Shims may also be used as spacers to fill gaps between parts subject to wear.

Image - http://www.irisclasson.com/wp-content/uploads/2013/01/shims.jpg