Octavo (sometimes abbreviated to “8vo,” and sometimes pronounced “eight-vo"), a term from bibliography, refers to the format of a book. An octavo is smaller than the folio and the quarto, but larger than the duodecimo. A sheet is folded over three times, so each sheet consists of eight leaves (or sixteen pages). The average hardcover book today is the rough size of an octavo.
Whereas the most serious materials (such as Bibles) have traditionally appeared in folios, more popular works (such as novels) have traditionally appeared in octavos.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.