The Collaborative International Dictionary of English v.0.48:

eightvo \eightvo\ n.
   the size of a book (ca. 16 x 23 cm) whose pages are made by
   folding a sheet of paper three times to form eight leaves.
   The pages have about half the area of a quarto. Symbolized
   as 8vo, and 8[deg].

   Syn: octavo, 8vo, 8[deg].
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

eightvo
    n 1: the size of a book whose pages are made by folding a sheet
         of paper three times to form eight leaves [syn: octavo,
         eightvo, 8vo]