noun, plural oc·to·dec·i·mos for 2.

a book size of about 4 × 6 1/4 inches (10 × 16 cm), determined by printing on sheets folded to form 18 leaves or 36 pages. Symbol: 18mo, 18°
a book of this size.


in octodecimo.

Origin of octodecimo

1855–60; short for New Latin in octōdecimō in an eighteenth (of a sheet)
Also called eighteenmo.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for octodecimo

Historical Examples of octodecimo

  • Other sizes occasionally used are called “sixteenmo” or “sextodecimo,” “eighteenmo” or “octodecimo,” etc.

    The Story of Books

    Gertrude Burford Rawlings

British Dictionary definitions for octodecimo


noun plural -mos

bookbinding another word for eighteenmo

Word Origin for octodecimo

C18: from New Latin phrase in octodecimo in an eighteenth (of a whole sheet)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012