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