Source : Free On-Line Dictionary of Computing

boot block
     
         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)