An excerpt of a larger publication that has been printed and bound separately for promotional purposes.
For example, publishers will print and bound a chapter of a book to send to booksellers or for the author to give away before the entire book is published.
Scholarly excerpts are another example; a portion of a large journal piece printed for a professor to distribute.
Offprints are highly sought after by collectors because, technically, they can be considered a first separate edition of the work and will often have a presentation inscription.