From 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]
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