n. pl. sex·to·dec·i·mos
1. The page size of a book composed of printer's sheets folded into 16 leaves or 32 pages.
2. A book composed of sextodecimo pages. In both senses also called sixteenmo.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.