(n.) 1. A series of different key sections which may be used to expand a master key system by repeating bittings on additional key sections. The keys of one key section will not enter the keyway of another key section. This type of system always includes another key section which will enter more than one, or all of the keyways.
(n.) 2. A keying system which uses such keyways and key sections.

Source: Lock Industry Standards and Training (LIST) Council