n. pl. eight·een·mos
[From 18mo, abbr. of octodecimo.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
n, pl -mos
1. (Printing, Lithography & Bookbinding) Also called: octodecimo a book size resulting from folding a sheet of paper into 18 leaves or 36 pages. Often written: 18mo or 18°
2. (Printing, Lithography & Bookbinding) a book of this size
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014