The Free On-line Dictionary of Computing (30 December 2018):

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)