Glossary of Terms
The portion of a jacket that folds into the book from front to back (and sometimes from top to bottom, see French Fold Jacket). A softcover book can also have flaps that fold inward on the front and back cover.
Flaps give extra space where content can be added, such as author information, a book synopsis, or promotional material.
Flaps are sometimes referred to as gatefold flaps, not to be confused with an inserted gatefold (which is either tipped in or bound into a book block).