(n.) 1. A metal ball used in the pin stack to accomplish some types of hotel or construction keying.
(n.) 2. A ball, usually made of steel, used by some lock manufacturers as the bottom element in the pin stack in one or more pin chambers.
(n.) 3. Any metal ball used as a tumbler’s primary component.
(n.) 4. Used as a means of repairing a drilled hole in a safe when welded in place.

Source: Lock Industry Standards and Training (LIST) Council