A suffix added to the names of certain numerals or to the
   numerals themselves, to indicate the number of leaves made by folding a
   sheet of paper; as, sixteenmo or 16mo; eighteenmo or 18mo. It is taken
   from the Latin forms similarly used; as, duodecimo, sextodecimo, etc. A
   small circle, placed after the number and near its top, is often used
   for -mo; as, 16¡, 18¡, etc.