What is "octodecimo"

Octodecimo \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.

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.

n. 1 (context paper printing English) A size of a sheet of paper resulting from folding and cutting a sheet of paper into eighteenths (3.5"-4.5" x 4.9"-5.9"). 2 (context printing English) A book consisting of pages of that size.