Octavo, a Latin word meaning "in eighth" or "for the eighth time",[1] (abbreviated 8vo, , or In-8) is a technical term describing the format of a book, which refers to the size of leaves produced from folding a full sheet of paper on which multiple pages of text were printed to form the individual sections (or gatherings) of a book. An octavo is a book or pamphlet made up of one or more full sheets (e.g. of A2 paper) on which 16 pages of text were printed, which were then folded three times to produce eight leaves. Each leaf of an octavo book thus represents one eighth the size of the original sheet. Other common book formats are folios and quartos. Octavo is also used as a general description of the size of books that are about 8 to 10 inches (200 to 250 millimetres) tall (almost A4 paper size), and as such does not necessarily indicate the actual printing format of the books, which may even be unknown as is the case for many modern books. These terms are discussed in greater detail in book sizes.

Octavo metrics compared to the folio and quarto.