Address mark
A special bit pattern on an FM- or MFM-formatted disk that tells the controller where a sector ID or a data block begins.
A disk controller can't simply count bits — it needs unmistakable landmarks in the stream. Address marks are byte patterns with deliberate clock violations: missing flux transitions that a correct MFM encoding would never produce, which makes them impossible to confuse with ordinary data. Two kinds exist: the ID address mark precedes the sector header (cylinder, head, sector, size), and the data address mark precedes the payload itself.
IBM's System/34 floppy format from 1977 set the pattern the rest of the industry inherited. A special "deleted data address mark" could even flag a sector as logically erased without overwriting it. It's on these marks that the data separator re-finds byte boundaries after every gap.