The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]The Collaborative International Dictionary of English v.0.48:
Eighteenmo \Eight`een"mo\, a. & n. See Octodecimo. [1913 Webster]