(1) A feature that can be connected to a system to provide additional storage and processing capacity.
(2) A feature that contains I/O hardware such as cards, tapes, and disk units. The system may have multiple expansion units. An expansion unit can be bolted directly to the side of a system and is contained within the system covers. An expansion unit can also be mounted on top of a rack and has its own covers. Internal signal cables interconnect the expansion unit to the system or rack.
(3) A machine type or feature that can be connected to a system unit to provide additional storage and processing capacity. This expansion unit may contain I/O hardware such as cards, tapes, and disk drives.