The Collaborative International Dictionary of English v.0.48:

Overture \O"ver*ture\, [OF. overture, F. ouverture, fr. OF.
   ovrir, F. ouvrir. See Overt.]
   1. An opening or aperture; a recess; a chamber. [Obs.]
      --Spenser. "The cave's inmost overture." --Chapman.
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   2. Disclosure; discovery; revelation. [Obs.]
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            It was he
            That made the overture of thy treasons to us.
                                                  --Shak.
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   3. A proposal; an offer; a proposition formally submitted for
      consideration, acceptance, or rejection. "The great
      overture of the gospel." --Barrow.
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   4. (Mus.) A composition, for a full orchestra, designed as an
      introduction to an oratorio, opera, or ballet, or as an
      independent piece; -- called in the latter case a concert
      overture.
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