Source : Free On-Line Dictionary of Computing

boot disk
     
         The {magnetic disk} (usually a {hard disk})
        from which an {operating system} {kernel} is loaded (or
        "bootstrapped").  This second phase in system start-up is
        performed by a simple bootstrap loader program held in {ROM},
        possibly configured by data stored in some form of writable
        {non-volatile storage}.
     
        {MS-DOS} and {Microsoft Windows} can be configured (in the
        {BIOS}) to try to boot off either {floppy disk} or {hard
        disk}, in either order.  By default they first check for the
        presence of a {floppy disk} in the drive at start-up and try
        to use that as a boot disk if present.  If no disk is in the
        drive they then try to boot off the hard disk.
     
        Some {operating systems}, notably {SunOS} and {Solaris}, can
        be configured to boot from a network rather than from disk.
        Such a system can thus run as a {diskless workstation}.
     
        (1997-06-09)