(ˈsɪksməʊ) n, pl-mos1. (Printing, Lithography & Bookbinding) Also called: sexto a book size resulting from folding a sheet of paper into six leaves or twelve pages, each one sixth the size of the sheet. Often written: 6moor6°2. (Printing, Lithography & Bookbinding) a book of this sizeAcronymsSee6MO