(1) The address of a particular device, specified at the time a system is installed.
(2) In mainframe computing, the address associated with a device on a given control unit. On ESCON or FICON interfaces, the unit address is the same as the device address. On OEMI interfaces, the unit address specifies a control unit and device pair on the interface.
(3) The identifier for a logical subsystem and the logical device within the subsystem.