octodecimo

Etymology
From ablative of.

Noun

 * 1)  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)  A book consisting of pages of that size.

Synonyms

 * 18mo, eighteenmo, 18º
 * 18mo, eighteenmo, 18º, T