from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The page size, 4 by 6 1/2 inches, of a book composed of printer's sheets folded into 18 leaves or 36 pages.
- n. A book composed of octodecimo pages.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A size of a sheet of paper resulting from folding and cutting a sheet of paper into eighteenths (3.5"-4.5" x 4.9"-5.9").
- n. A book consisting of pages of that size.
from the GNU version of the Collaborative International Dictionary of English
- adj. Having eighteen leaves to a sheet
- n. A book composed of sheets each of which is folded into eighteen leaves; hence; indicating more or less definitely a size of book, whose sheets are so folded; -- usually written 18mo or 18°, and called eighteenmo.
from The Century Dictionary and Cyclopedia
- n. Same as eighteenmo. Abbreviated 18mo.
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 dekm̥ in Indo-European roots.(American Heritage® Dictionary of the English Language, Fourth Edition)
From Latin ablative of octusdecimus ("eighteenth") (Wiktionary)