oc·to·dec·i·mo 
(ŏk′tə-dĕsə-mō′)
(ŏk′tə-dĕsə-mō′)n. pl. oc·to·dec·i·mos
1. The page size, 4 by 6.5 inches, of a book composed of printer's sheets folded into 18 leaves or 36 pages.
2. A book composed of octodecimo pages. In both senses also called eighteenmo.
[Late Latin (in) octōdecimō, (in) an eighteenth, ablative sing. of octōdecimus, eighteenth, from Latin octōdecim, eighteen : octō, eight; see oktō(u) in the Appendix of Indo-European roots + decem, ten; see dek in the Appendix of Indo-European roots.]