Logical Unit Number
A term used by the SCSI specification to describe a particular sub-system in a multi function device. A multi function device such as X's Drive II has multiple LUNs. The individual LUNs are assigned to the separate card slots and hard disks. LUN 0 is the built in hard disk, LUN 1 is the Smart Media slot, LUN 2 is the MMC card slot and so on.