nSextodecimoA 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°.
aSextodecimoHaving 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.
Century Dictionary and Cyclopedia
nsextodecimoA sheet of paper when regularly folded in 16 leaves of equal size; also, a pamphlet or book made up of folded sheets of 16 leaves: usually indicated thus, 16 mo or 16° (commonly read sixteenmo). Also used adjectively. When the size of paper is not named, the 16mo leaf untrimmed is supposed to be of the size 4½ by 6⅞ inches. Also decimosexto.
Webster's Revised Unabridged Dictionary
L. sextus-decimus, the sixteenth; sextus, the sixth (fr. sex, six) + decimus, the tenth, from decem, ten. See -mo