(1):

(a.) 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.

(2):

(n.) 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¡.