What is "eighteenmo"

Octodecimo \Oc`to*dec"i*mo\, n.; pl. Octodecimos. 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[deg], and called eighteenmo.

n. octodecimo (as a paper size in printing).