Source : WordNet®

disk controller
     n : (computer science) a circuit or chip that translates
         commands into a form that can control a hard disk drive

Source : Free On-Line Dictionary of Computing

disk controller
     
         (Or "hard disk controller", HDC) The
        circuit which allows the {CPU} to communicate with a {hard
        disk}, {floppy disk} or other kind of {disk drive}.
     
        The most common disk controllers in use are {IDE} and {SCSI}
        controllers.  Most home {personal computers} use IDE
        controllers.  High end PCs, {workstations} and network {file
        servers} mostly have {SCSI adaptors}.
     
        (1998-03-16)