The Collaborative International Dictionary
Sextodecimo \Sex`to*dec"i*mo\, a. [L. sextus-decimus the sixteenth; sextus the sixth (fr. sex six) + decimus the tenth, from decem ten. See -mo.] Having sixteen leaves to a sheet; of, or equal to, the size of one fold of a sheet of printing paper when folded so as to make sixteen leaves, or thirty-two pages; as, a sextodecimo volume.
Sextodecimo \Sex`to*dec"imo\, n.; pl. Sextodecimos. A book composed of sheets each of which is folded into sixteen leaves; hence, indicating, more or less definitely, a size of a book; -- usually written 16mo, or 16[deg].
Wiktionary
n. 1 (context paper printing English) A size of a sheet of paper resulting from folding and cutting a sheet of paper into sixteenths (3.25"-5" x 5"-6.25"). 2 (context printing English) A book consisting of pages of that size.
Usage examples of "sextodecimo".
Some of the volumes I could see were massive, like quarried slabs, and were attached to the shelves by long chains that hung down like necklaces from their wooden bindings, while others, tiny sextodecimos, were no larger than snuff-boxes, small enough to fit in the palm of the hand, their pasteboard covers tied with faded ribbons or locked with tiny clasps.