\Oc`todec"i*mo\, a. [L. octodecim eighteen. See
{Octavo}, {Decimal}, and {-mo}.]
Having eighteen leaves to a sheet; as, an octodecimo form,
book, leaf, size, etc.

\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.