(n.) 1. A projection on a lock bolt which prevents movement of the bolt unless it can enter gates of properly aligned tumblers.
(n.) 2. Any locking element other than a sidebar or shackle designed to enter a tumbler’s gate.
(n.) 3. A projection on the lever in a combination lock that prevents the lever from engaging the drive cam until the proper combination is entered.

Source: Lock Industry Standards and Training (LIST) Council

See also: “sidebar”