In binding and finishing, the portion of a book's jacket which folds under the cloth cover holding the jacket on the book. Book flaps often contain promotional copy and author information.
In copy preparation, a flap is an overlay containing type (or other page element), used in conjunction with the type (or other page element) on the mechanical beneath the flap. Flaps are used when two pages have common elements; the mechanical (with the flap up) is photographed for one page, and photographed again (with the flap down) for the second page.