a. Having eighteen leaves to a sheet; as, an octodecimo
form, book, leaf, size, etc.
n. A book composed of sheets each of which is folded into
eighteen leaves; hence; indicating more or less definitely a size of
book, whose sheets are so folded; -- usually written 18mo or 18�, and
called eighteenmo.
n. 十八开纸, 十八开