The Collaborative International Dictionary of English v.0.48:

background \back"ground`\, n. [Back, a. + ground.]
   [1913 Webster]
   1. Ground in the rear or behind, or in the distance, as
      opposed to the foreground, or the ground in front.
      [1913 Webster]

   2. (Paint.) The space which is behind and subordinate to a
      portrait or group of figures.
      [1913 Webster]

   Note: The distance in a picture is usually divided into
         foreground, middle distance, and background.
         --Fairholt.
         [1913 Webster]

   3. Anything behind, serving as a foil; as, the statue had a
      background of red hangings.
      [1913 Webster]

   4. A place in obscurity or retirement, or out of sight.
      [1913 Webster]

            I fancy there was a background of grinding and
            waiting before Miss Torry could produce this highly
            finished . . . performance.           --Mrs.
                                                  Alexander.
      [1913 Webster]

            A husband somewhere in the background. --Thackeray.
      [1913 Webster]

   5. The set of conditions within which an action takes place,
      including the social and physical conditions as well as
      the psychological states of the participants; as, within
      the background of the massive budget deficits of the
      1980's, new spending programs had little chance of passage
      by the congress.
      [PJC]

   6. The set of conditions that precede and affect an action,
      such as the social and historical precedents for the
      event, as well as the general background[5]; as, against
      the background of their expulsion by the Serbs, the desire
      of Kosovars for vengeance is understandable though
      regrettable.
      [PJC]

   7. (Science) The signals that may be detected by a
      measurement which are not due to the phenomenon being
      studied, and tend to make the measurement uncertain to a
      greater or lesser degree. Specifically: (Physics)
      Electronic noise present in a system using electronic
      measuring instrument or in a telecommunications system,
      which may hide and which must be differentiated from the
      desired signal; also called background noise or noise.
      [PJC]

   8. (Journalism) An agreement between a journalist and an
      interviewee that the name of the interviewee will not be
      quoted in any publication, although the substance of the
      remarks may be reported; -- often used in the phrase "on
      background". Compare deep background.
      [PJC]

   To place in the background, to make of little consequence.

   To keep in the background, to remain unobtrusive,
      inconspicuous or out of sight; -- of people.

   deep background, (Journalism) the status of an interview
      which must not be quoted in a publication, even without
      attribution. Compare background[8].
      [1913 Webster +PJC]