boot block

from The Free On-line Dictionary of Computing (8 July 2008)

boot block

   <operating system> A program at some fixed location on a {hard
   disk}, {floppy disk} or other media, which is loaded when the
   computer is turned on or rebooted and which controls the next
   phase of loading the actual {operating system}.  The loading
   and execution of the boot block is usually controlled by
   {firmware} in {ROM} or {PROM}.

   (1997-07-03)