'Leader tape' is the term used in the recording
industries for the first few feet of protective, passive tape on the outside of a reel
or recording tape
The main purpose of the leader tape is to protect the magnetic (or optical) recording medium from mechanical stress, resulting from the operations of loading the reel, and threading the tape. Typically, a tape (film) would be loaded and threaded many times for playback (or projection). The leader has different physical properties to the recording medium, which would be more prone to stretching and snapping if the sacrificial leader was not there.
With a movie, the leader tape is optically opaque. The leader to a magnetic tape needs merely to be magnetically passive, and is usually coloured white.
Leader tapes are not used with computer magtapes, as the loading and threading operation is performed by machine, hence not subject to the same stresses as manual threading. Magtapes have an optical tape mark, which indicates the beginning of tape (BOT).
Audio cassettes do contain a short leader tape at the beginning of each side; this is present for protection at the end of winding, and during transportation (tip: always rewind your cassettes before transporting them).
VHS Video cassettes contain short leaders which do not become exposed. They have an optical tape mark, by way of using a transparent leader, which is detected while winding (thanks to pokey for pointing this out).