Short for beginning of tape marker. A physical feature of a magnetic tape by which the tape transport senses on the tape the start of the volume into which data can be recorded, or has been recorded. When the transport senses the marker it commences the logical sequence of recording or reading data. The marker may be, for example, a rectangular strip of reflective material adhering to the tape, a transparent part of the tape, or a hole in the tape. See also EOT marker. Compare tape mark.