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. [ 1913 Webster ]

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°. [ 1913 Webster ]