The page size, 4 by 6.5 inches, of a book composed of printer's sheets folded into 18 leaves or 36 pages.
A book composed of octodecimo pages.
A page size (about 4 by 612 in),118 of a printer's sheet.
A book with pages of this size.
With pages of this size.
(printing) A book consisting of pages of that size.
Origin of octodecimo
From American Heritage Dictionary of the English Language, 5th Edition