Source : Webster's Revised Unabridged Dictionary (1913)

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].