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]

WordNet (r) 3.0 (2006):

background
    n 1: a person's social heritage: previous experience or
         training; "he is a lawyer with a sports background"
    2: the part of a scene (or picture) that lies behind objects in
       the foreground; "he posed her against a background of rolling
       hills" [syn: background, ground]
    3: information that is essential to understanding a situation or
       problem; "the embassy filled him in on the background of the
       incident" [syn: background, background knowledge]
    4: extraneous signals that can be confused with the phenomenon
       to be observed or measured; "they got a bad connection and
       could hardly hear one another over the background signals"
       [syn: background, background signal]
    5: relatively unimportant or inconspicuous accompanying
       situation; "when the rain came he could hear the sound of
       thunder in the background"
    6: the state of the environment in which a situation exists;
       "you can't do that in a university setting" [syn: setting,
       background, scope]
    7: (computer science) the area of the screen in graphical user
       interfaces against which icons and windows appear [syn:
       background, desktop, screen background]
    8: scenery hung at back of stage [syn: backdrop, background,
       backcloth]
    v 1: understate the importance or quality of; "he played down
         his royal ancestry" [syn: background, play down,
         downplay] [ant: foreground, highlight, play up,
         spotlight]

Moby Thesaurus II by Grady Ward, 1.0:

155 Moby Thesaurus words for "background":
   action, agora, amphitheater, anagnorisis, angle, architectonics,
   architecture, arena, argument, athletic field, atmosphere,
   auditorium, back, backdrop, background detail, backstage,
   bear garden, behind the scenes, blaseness, bowl, boxing ring,
   breeding, bull ring, campus, canvas, catastrophe, characterization,
   circus, cockpit, coliseum, color, colosseum, complication,
   continuity, contrivance, course, credentials, curriculum vitae,
   decorative composition, decorative style, denouement, design,
   detail, development, device, distance, episode, experience, fable,
   falling action, family, field, figure, floor, foil,
   foreground detail, form, forum, gimmick, ground, gym, gymnasium,
   hall, hinterland, hippodrome, history, horizon, in the background,
   incident, inconspicuous, line, lists, local color, locale,
   marketplace, mat, milieu, mise-en-scene, mood, motif, movement,
   mythos, national style, obscurity, offing, open forum,
   ornamental motif, palaestra, parade ground, past experience,
   pattern, period style, peripeteia, pit, place, plan, platform,
   plot, practical knowledge, practice, precinct, prize ring,
   public square, purlieu, range, rear, recognition, remote distance,
   repeated figure, ring, rising action, sagacity, scene,
   scene of action, scenery, scheme, seasoning, secondary plot,
   setting, site, slant, sophistication, sphere, squared circle,
   stadium, stage, stage set, stage setting, story, structure, style,
   subject, subplot, switch, tempering, terrain, the distance,
   theater, thematic development, theme, tilting ground, tiltyard,
   tone, topic, touch, training, twist, unnoticed, unobtrusive,
   unseen, upbringing, vanishing point, walk, worldly wisdom,
   wrestling ring



The Jargon File (version 4.4.7, 29 Dec 2003):

background
 n.,adj.,vt.

    [common] To do a task in background is to do it whenever foreground
    matters are not claiming your undivided attention, and to background
    something means to relegate it to a lower priority. ?For now, we'll just
    print a list of nodes and links; I'm working on the graph-printing problem
    in background.? Note that this implies ongoing activity but at a reduced
    level or in spare time, in contrast to mainstream ?back burner? (which
    connotes benign neglect until some future resumption of activity). Some
    people prefer to use the term for processing that they have queued up for
    their unconscious minds (a tack that one can often fruitfully take upon
    encountering an obstacle in creative work). Compare amp off, slopsucker
    .

    Technically, a task running in background is detached from the terminal
    where it was started (and often running at a lower priority); oppose 
    foreground. Nowadays this term is primarily associated with Unix, but it
    appears to have been first used in this sense on OS/360.


The Free On-line Dictionary of Computing (30 December 2018):

background

   1.  A task running in the background (a
   background task) is detached from the terminal where it was
   started (and often running at a lower priority); opposite of
   foreground.  This means that the task's input and output
   must be from/to files (or other processes).

   Nowadays this term is primarily associated with Unix, but it
   appears to have been first used in this sense on OS/360.

   Compare amp off, batch, slopsucker.

   2.  For a human to do a task "in the background" is to
   do it whenever foreground matters are not claiming your
   undivided attention, and "to background" something means to
   relegate it to a lower priority.  "For now, we'll just print a
   list of nodes and links; I'm working on the graph-printing
   problem in the background."  Note that this implies ongoing
   activity but at a reduced level or in spare time, in contrast
   to mainstream "back burner" (which connotes benign neglect
   until some future resumption of activity).  Some people prefer
   to use the term for processing that they have queued up for
   their unconscious minds (often a fruitful tack to take upon
   encountering an obstacle in creative work).

   (1996-05-28)