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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].