A black, opaque film, usualyl colled simply a black leader. It is what the negative cutter uses when preparing A&B rolls. It is very important that it be emulsion leader rather than plastic leader when used for A&B rolls, since plastic leader cannot be cement spliced. It also must be very opaque, not any black piece of film will do.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission