The Collaborative International Dictionary of English v.0.48:

Indifference \In*dif"fer*ence\, n. [L. indifferentia similarity,
   lack of difference: cf. F. indiff['e]rence.]
   [1913 Webster]
   1. The quality or state of being indifferent, or not making a
      difference; lack of sufficient importance to constitute a
      difference; absence of weight; insignificance.
      [1913 Webster]

   2. Passableness; mediocrity.
      [1913 Webster]

   3. Impartiality; freedom from prejudice, prepossession, or
      bias.
      [1913 Webster]

            He . . . is far from such indifference and equity as
            ought and must be in judges which he saith I assign.
                                                  --Sir T. More.
      [1913 Webster]

   4. Absence of anxiety or interest in respect to what is
      presented to the mind; unconcernedness; as, entire
      indifference to all that occurs.
      [1913 Webster]

            Indifference can not but be criminal, when it is
            conversant about objects which are so far from being
            of an indifferent nature, that they are highest
            importance.                           --Addison.

   Syn: Carelessness; negligence; unconcern; apathy;
        insensibility; coldness; lukewarmness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

indifference
    n 1: unbiased impartial unconcern
    2: apathy demonstrated by an absence of emotional reactions
       [syn: emotionlessness, impassivity, impassiveness,
       phlegm, indifference, stolidity, unemotionality]
    3: the trait of lacking enthusiasm for or interest in things
       generally [syn: apathy, indifference, numbness,
       spiritlessness]
    4: the trait of remaining calm and seeming not to care; a casual
       lack of concern [syn: nonchalance, unconcern,
       indifference]

Moby Thesaurus II by Grady Ward, 1.0:

265 Moby Thesaurus words for "indifference":
   Laodiceanism, abandon, abeyance, absentmindedness, abstraction,
   abulia, acedia, alienation, aloofness, anxiety, anxiety equivalent,
   anxiety state, apathy, ataraxia, ataraxy, benumbedness, blah,
   blahs, boredom, callousness, careless abandon, carelessness,
   casualness, catalepsy, catatonia, catatonic stupor, coldness,
   comatoseness, compulsion, coolness, cursoriness, deadliness,
   deathliness, dejection, depression, detachment, disinterest,
   disinterestedness, dispassion, dispassionateness, disregard,
   disregardfulness, distraction, dormancy, drowsiness, dullness,
   easiness, easygoingness, elation, emotionalism, enervation, ennui,
   entropy, equitability, equitableness, euphoria, evenhandedness,
   fairishness, fairness, fatigue, flightiness, flippancy,
   folie du doute, forgetfulness, frivolousness, giddiness, hastiness,
   heartlessness, heaviness, hebetude, heedlessness, hopelessness,
   hypochondria, hysteria, hysterics, immateriality, impartiality,
   impassiveness, impassivity, impotence, imprecision, inadvertence,
   inadvertency, inanimation, inappetence, inattention,
   inattentiveness, inconsequence, inconsequentiality,
   inconsiderableness, inconsiderateness, inconsideration,
   incuriosity, incuriousness, indevotion, indevoutness,
   indifferentism, indifferentness, indolence, ineffectuality,
   inertia, inertness, inferiority, inobservance, insensibility,
   insignificance, insouciance, intellectual inertia, irrelevance,
   jadedness, lack of appetite, lack of foresight, lack of interest,
   lackadaisicalness, lackluster, languidness, languishment, languor,
   languorousness, lassitude, latency, laxity, laxness, laziness,
   leniency, lenitude, lentor, lethargicalness, lethargy, levity,
   lifelessness, lightmindedness, listlessness, littleness, looseness,
   loosening, lotus-eating, low priority, lukewarm piety, mania,
   marginality, mediocreness, mediocrity, melancholia,
   mental distress, middlingness, moderateness, modestness, modesty,
   negligence, negligibility, neutrality, nonchalance, nonobservance,
   nonreligiousness, numbness, objectivity, oblivion, obliviousness,
   obsession, offhandedness, oscitancy, overindulgence,
   overpermissiveness, passableness, passiveness, passivity,
   pathological indecisiveness, perfunctoriness, permissiveness,
   pettiness, phlegm, phlegmaticalness, phlegmaticness, picayune,
   picayunishness, plucklessness, pococurantism, pokiness,
   preoccupation, psychalgia, psychomotor disturbance, puniness,
   recklessness, regardlessness, relaxation, relaxedness, remissness,
   resignation, resignedness, respectability, satedness,
   secondariness, shallowness, slackness, sleepiness, slightness,
   sloppiness, sloth, slothfulness, slowness, sluggishness, smallness,
   softness, somnolence, sopor, soporifousness, spiritlessness,
   sprezzatura, spunklessness, stagnancy, stagnation, stasis,
   stolidity, stupefaction, stupor, superficiality, supineness,
   suspense, tactlessness, tediousness, thoughtlessness, tic,
   tolerableness, torpidity, torpidness, torpitude, torpor,
   triviality, twitching, unalertness, unawareness, unbiasedness,
   unconcern, unconsciousness, undevoutness, undutifulness,
   unheedfulness, unimportance, unimpressiveness, uninquisitiveness,
   unintentiveness, uninterestedness, unmindfulness, unnoteworthiness,
   unobservance, unopinionatedness, unprejudicedness, unpreparedness,
   unreadiness, unreligiousness, unresponsiveness, unrestraint,
   unsolicitousness, unsolicitude, unthinkingness, unwariness,
   unwatchfulness, vegetation, vis inertiae, weakness, weariness,
   withdrawal, withdrawnness, world-weariness