Octodecimo

Part of speech: noun (n.)

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 18º, and called eighteenmo.

Part of speech: adjective (a.)

Having eighteen leaves to a sheet; as, an octodecimo form, book, leaf, size, etc.