1911 Encyclopædia Britannica/Octavo
OCTAVO, a shortened form of Lat. in octavo, “in an eighth, i.e. of a sheet of paper, a term applied to a size of paper and to a size of a printed volume. Paper is in octavo when a whole single sheet is folded three times to form eight leaves; a book is technically termed of “octavo” size when made up of sheets folded three times (see Bibliography and Paper).