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]
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