The Collaborative International Dictionary of English v.0.48:

Octavo \Oc*ta"vo\, n.;pl. Octavos. [L. in octavo; in in +
   octavo, abl. of octavus. See Octave.]
   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[deg].
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Octavo \Oc*ta"vo\, a.
   Having eight leaves to a sheet; as, an octavo form, book,
   leaf, size, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

octavo
    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]