The first sector on any disk. It's called the boot sector because the PC BIOS checks to see if the last two bytes of it are 0x55AA and if so hands control off to any code that happens to be in that sector, normally this code loads the OS off the rest of the disk and starts it, although floppies without an OS on them usually display an error message asking you to replace the disk.
One page links to BootSector:
