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°, and called eighteenmo. [ 1913 Webster ]

a. [ L. octodecim eighteen. See Octavo, Decimal, and -mo. ] Having eighteen leaves to a sheet; as, an octodecimo form, book, leaf, size, etc. [ 1913 Webster ]