O·ver·ture
  1. An opening or aperture; a recess; a chamber. [Obs.] --Spenser. “The cave's inmost overture.”
  2. Disclosure; discovery; revelation. [Obs.]
  It was he
  That made the overture of thy treasons to us.   --Shak.
  3. A proposal; an offer; a proposition formally submitted for consideration, acceptance, or rejection. “The great overture of the gospel.”
  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.