eight·een·mo (ā-tēn'mō) n.
pl.eight·een·mos See octodecimo.
[From 18mo, abbr. of octodecimo.]
oc·to·dec·i·mo (ŏk'tə-děs'ə-mō') n.
pl.oc·to·dec·i·mos In both senses also called eighteenmo.
The page size, 4 by 6 1/2 inches, of a book composed of printer's sheets folded into 18 leaves or 36 pages.
A book composed of octodecimo pages.
[Late Latin (in) octōdecimō, (in) an eighteenth, ablative sing. of octōdecimus, eighteenth, from Latin octōdecim, eighteen : octō, eight; see oktō(u) in Indo-European roots + decem, ten; see dek in Indo-European roots.]