The Collaborative International Dictionary of English v.0.48:

Abstraction \Ab*strac"tion\, n. [Cf. F. abstraction. See
   Abstract, a.]
   1. The act of abstracting, separating, or withdrawing, or the
      state of being withdrawn; withdrawal.
      [1913 Webster]

            A wrongful abstraction of wealth from certain
            members of the community.             --J. S. Mill.
      [1913 Webster]

   2. (Metaph.) The act process of leaving out of consideration
      one or more properties of a complex object so as to attend
      to others; analysis. Thus, when the mind considers the
      form of a tree by itself, or the color of the leaves as
      separate from their size or figure, the act is called
      abstraction. So, also, when it considers whiteness,
      softness, virtue, existence, as separate from any
      particular objects.
      [1913 Webster]

   Note: Abstraction is necessary to classification, by which
         things are arranged in genera and species. We separate
         in idea the qualities of certain objects, which are of
         the same kind, from others which are different, in
         each, and arrange the objects having the same
         properties in a class, or collected body.
         [1913 Webster]

               Abstraction is no positive act: it is simply the
               negative of attention.             --Sir W.
                                                  Hamilton.
         [1913 Webster]

   3. An idea or notion of an abstract, or theoretical nature;
      as, to fight for mere abstractions.
      [1913 Webster]

   4. A separation from worldly objects; a recluse life; as, a
      hermit's abstraction.
      [1913 Webster]

   5. Absence or absorption of mind; inattention to present
      objects.
      [1913 Webster]

   6. The taking surreptitiously for one's own use part of the
      property of another; purloining. [Modern]
      [1913 Webster]

   7. (Chem.) A separation of volatile parts by the act of
      distillation. --Nicholson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

abstraction
    n 1: a concept or idea not associated with any specific
         instance; "he loved her only in the abstract--not in
         person" [syn: abstraction, abstract]
    2: the act of withdrawing or removing something
    3: the process of formulating general concepts by abstracting
       common properties of instances [syn: abstraction,
       generalization, generalisation]
    4: an abstract painting
    5: preoccupation with something to the exclusion of all else
       [syn: abstractedness, abstraction]
    6: a general concept formed by extracting common features from
       specific examples [syn: abstraction, abstract entity]

Moby Thesaurus II by Grady Ward, 1.0:

199 Moby Thesaurus words for "abstraction":
   Walter Mitty, ablation, abrasion, absence of mind,
   absentmindedness, absorption, abstract, abstract idea,
   abstractedness, abulia, alienation, altarpiece, analysis,
   annexation, anxiety, anxiety equivalent, anxiety state, apathy,
   appropriation, bemusement, block print, boosting, bromide,
   brown study, castle-building, catatonic stupor, cliche,
   close study, collage, color print, commonplace, compulsion,
   concentration, contemplativeness, conversion, conveyance, copy,
   cyclorama, daub, daydream, daydreamer, daydreaming, deduction,
   deep thought, dejection, depression, depth of thought, detachment,
   diptych, disarticulation, disassociation, disconnectedness,
   disconnection, discontinuity, disengagement, disjointing,
   disjunction, dislocation, disunion, division, divorce, divorcement,
   doctrinairism, doctrinality, doctrinarity, dream, dreaming,
   elation, embezzlement, emotionalism, engraving, engrossment,
   erosion, euphoria, explanation, fantasy, fantasying, filching,
   fit of abstraction, folie du doute, fraud, fresco, general idea,
   generalization, generalized proposition, glittering generality,
   graft, hackneyed expression, hypochondria, hysteria, hysterics,
   icon, illumination, illustration, image, incoherence, indifference,
   insensibility, isolation, lethargy, liberation, lieu commun,
   lifting, likeness, locus communis, luxation, mania, melancholia,
   melancholy, mental distress, mere theory, miniature, montage,
   mooning, moonraking, mosaic, mural, muse, musefulness, musing,
   muted ecstasy, obsession, panorama, parting, partition,
   pathological indecisiveness, pensiveness, photograph, picture,
   pilferage, pilfering, pinching, pipe dream, pipe-dreaming,
   platitude, poaching, preoccupation, print, profound thought,
   psychalgia, psychomotor disturbance, purification, refinement,
   reflectiveness, removal, representation, reproduction, reverie,
   scrounging, segmentation, separation, separatism, shoplifting,
   snatching, sneak thievery, snitching, speculation, speculativeness,
   stained glass window, stargazing, stealage, stealing, stencil,
   still life, study, stupor, subdivision, subduction, sublation,
   subtraction, sweeping statement, swindle, swiping, tableau,
   taking away, tapestry, theft, theoretic, theoretical basis,
   theoretics, theoria, theoric, theorization, theory, thievery,
   thieving, thoughtfulness, tic, tired cliche, trance, triptych,
   truism, twitching, unresponsiveness, wall painting, wistfulness,
   withdrawal, woolgathering, zoning



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

abstraction

   1. Generalisation; ignoring or hiding details to capture some
   kind of commonality between different instances.  Examples are
   abstract data types (the representation details are hidden),
   abstract syntax (the details of the concrete syntax are
   ignored), abstract interpretation (details are ignored to
   analyse specific properties).

   2.  Parameterisation, making something a function
   of something else.  Examples are lambda abstractions (making
   a term into a function of some variable), higher-order
   functions (parameters are functions), bracket abstraction
   (making a term into a function of a variable).

   Opposite of concretisation.

   (1998-06-04)