Tape mark
A special recorded pattern that marks end-of-file on a tape — the tape's file separator, readable without understanding the contents.
A tape holds a stream of blocks with no directory. A tape mark (file mark) is a distinct recorded pattern, longer than a normal inter-block gap, that the drive reports as a special status rather than as data. Software reads until it hits one to find end-of-file; two in a row conventionally mean end-of-tape.
The Unix command mt still speaks this language — fsf (forward-space-file) skips forward N tape marks. tar (tape archive) and dump were built around the model: concatenate files, separate them with marks, and you can spool a whole backup to one reel and wind back to any file.