The Collaborative International Dictionary
Octodecimos
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.
Wiktionary
octodecimos
n. (plural of octodecimo English)