Shim

(1) Any thin, flat or curved piece of metal, capable of being inserted into a narrow or tight space. (2) A piece of metal which is used to enable the plug of a pin-tumbler cylinder to rotate, by inserting that piece of metal between the pin tumblers, in a technique known as ‘shimming open a cylinder.’ (3) A thin piece of plastic or metal which is inserted between a door and its frame, in order to push back a spring latch, thereby gaining entry