The Collaborative International Dictionary of English v.0.48:

Exposition \Ex`po*si"tion\, n. [L. expositio, fr. exponere,
   expositum: cf. F. exposition. See Expound.]
   1. The act of exposing or laying open; a setting out or
      displaying to public view.
      [1913 Webster]

   2. The act of expounding or of laying open the sense or
      meaning of an author, or a passage; explanation;
      interpretation; the sense put upon a passage; a law, or
      the like, by an interpreter; hence, a work containing
      explanations or interpretations; a commentary.
      [1913 Webster]

            You know the law; your exposition
            Hath been most sound.                 --Shak.
      [1913 Webster]

   3. Situation or position with reference to direction of view
      or accessibility to influence of sun, wind, etc.;
      exposure; as, an easterly exposition; an exposition to the
      sun. [Obs.] --Arbuthnot.
      [1913 Webster]

   4. A public exhibition or show, as of industrial and artistic
      productions; as, the Paris Exposition of 1878. [A
      Gallicism]
      [1913 Webster]

WordNet (r) 3.0 (2006):

exposition
    n 1: a systematic interpretation or explanation (usually
         written) of a specific topic [syn: exposition,
         expounding]
    2: a collection of things (goods or works of art etc.) for
       public display [syn: exhibition, exposition, expo]
    3: an account that sets forth the meaning or intent of a writing
       or discourse; "we would have understood the play better if
       there had been some initial exposition of the background"
    4: (music) the section of a movement (especially in sonata form)
       where the major musical themes first occur