The length of a book in pages, including the prelims. In printing terms, the extent equals the whole number of signatures to be printed (i.e. including any blank pages at the end of the last signature).
This definition is extracted (and expanded on) from the book Getting Published: A Companion for the Humanities and Social Sciences by Gerald Jackson and Marie Lenstrup. It was first referred to in the blog Getting Published in a post on calculating book length.