The Collaborative International Dictionary of English v.0.48:

Octave \Oc"tave\, n. [F., fr. L. octava an eighth, fr. octavus
   eighth, fr. octo eight. See Eight, and cf. Octavo,
   Utas.]
   1. The eighth day after a church festival, the festival day
      being included; also, the week following a church
      festival. "The octaves of Easter." --Jer. Taylor.
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   2. (Mus.)
      (a) The eighth tone in the scale; the interval between one
          and eight of the scale, or any interval of equal
          length; an interval of five tones and two semitones.
      (b) The whole diatonic scale itself.
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   Note: The ratio of a musical tone to its octave above is 1:2
         as regards the number of vibrations producing the
         tones.
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   3. (Poet.) The first two stanzas of a sonnet, consisting of
      four verses each; a stanza of eight lines.
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            With mournful melody it continued this octave. --Sir
                                                  P. Sidney.
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   Double octave. (Mus.) See under Double.

   Octave flute (Mus.), a small flute, the tones of which
      range an octave higher than those of the German or
      ordinary flute; -- called also piccolo. See Piccolo.
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   4. A small cask of wine, the eighth part of a pipe.
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