- the page size of a book made up of printer's sheets folded into eighteen leaves, each leaf being approximately 4 by 6 in
- a book consisting of pages of this size
Origin: eighteen plush -mo, as inch(es) octodecimo
Used by arrangement with John Wiley & Sons, Inc.
noun pl. eight·een·mos
Origin: From 18mo, abbr. of octodecimo.