oc·ta·vo /ɑkˈte(ˌ)vo, ˈtɑ-/
  八開紙,八開本
  Oc·ta·vo n.;pl. Octavos   A book composed of sheets each of which is folded into eight leaves; hence, indicating more or less definitely a size of book so made; -- usually written 8vo or 8°.
  Oc·ta·vo, a. Having eight leaves to a sheet; as, an octavo form, book, leaf, size, etc.
  ◄ ►
  octavo
       n : the size of a book whose pages are made by folding a sheet
           of paper three times to form eight leaves [syn: eightvo,
            8vo]