Definition of: octodecimo

(ok′tə·des′ə·mō) nounplural ·mos
The page size (approximately 4 × 6 ½ inches) of a book made up of printer's sheets folded into 18 leaves; also, a book consisting of pages of this size: often called eighteenmo and written 18mo or 18°.
—adjective In octodecimo; consisting of pages of this size. [<L (in) octodecimo (in) an eighteenth (of a sheet)]