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.
      [1913 Webster]

   2. Disclosure; discovery; revelation. [Obs.]
      [1913 Webster]

            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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Overture \O"ver*ture\, v. t.
   To make an overture to; as, to overture a religious body on
   some subject.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overture
    n 1: orchestral music played at the beginning of an opera or
         oratorio
    2: something that serves as a preceding event or introduces what
       follows; "training is a necessary preliminary to employment";
       "drinks were the overture to dinner" [syn: preliminary,
       overture, prelude]
    3: a tentative suggestion designed to elicit the reactions of
       others; "she rejected his advances" [syn: overture,
       advance, approach, feeler]