octodecimo

noun, plural oc·to·dec·i·mos for 2.

1. a book size of about 4 × 6 1/4 inches (10 × 16 cm), determined by printing on sheets folded to form 18 leaves or 36 pages. Symbol: 18mo, 18°
2. a book of this size.

adjective

3. in octodecimo.

noun plural -mos

1. bookbinding another word for eighteenmo