du·o·dec·i·mo (do?o??-d?s??-m??, dyo?o?-)n. pl. du·o·dec·i·mos 1. The size (5 by 7 3/4 inches) of book pages formed by folding single sheets from a printing press into 12 leaves each.2. A book composed of pages of this size. In both senses also called twelvemo.[Latin (in) duodecim?, (in) a twelfth, ablative of duodecimus, twelfth; see duodecimal.]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.duodecimo (?dju????d?s??m??) n, pl -mos1. (Printing, Lithography & Bookbinding) Also called: twelvemo a book size resulting from folding a sheet of paper into twelve leaves. Often written: 12mo or 12° 2. (Printing, Lithography & Bookbinding) a book of this size[C17: from Latin phrase in duodecim? in twelfth, from duodecim twelve]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014du?o?dec?i?mo (?du ??d?s ??mo?, ?dyu-) n., pl. -mos, adj. n. 1. a book size of about 5 x 7½ in. (13 x 19 cm), determined by printing on sheets folded to form 12 leaves or 24 pages. Symbol: 12 mo, 12° 2. a book of this size. adj. 3. in duodecimo; twelvemo. Also called twelvemo. [1650?60; short for Latin in duodecim? in twelfth] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.