A dust jacket is a protective cover that is usually made of paper and is placed over the cover of a book. It is designed to protect the book from dirt, dust, and wear and tear. The term 'dust jacket' is derived from the fact that it was originally made of cloth or paper that was coated with a dust-resistant material. The dust jacket also serves as a promotional tool, as it often contains information about the book, such as its title, author, publisher, and a brief synopsis.