n. pl. oc·to·dec·i·mos
1. The page size, 4 by 6.5 inches, of a book composed of printer's sheets folded into 18 leaves or 36 pages.
2. A book composed of octodecimo pages. In both senses also called eighteenmo.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.