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

boot block

    A program 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.  It may be at some fixed location possibly or may be
   pointed to by the master boot record.

   (2009-05-19)