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]