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°, and called eighteenmo.