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From 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]