The Collaborative International Dictionary of English v.0.48:

Apathy \Ap"a*thy\, n.; pl. Apathies. [L. apathia, Gr. ?; 'a
   priv. + ?, fr. ?, ?, to suffer: cf. F. apathie. See
   Pathos.]
   Want of feeling; privation of passion, emotion, or
   excitement; dispassion; -- applied either to the body or the
   mind. As applied to the mind, it is a calmness, indolence, or
   state of indifference, incapable of being ruffled or roused
   to active interest or exertion by pleasure, pain, or passion.
   "The apathy of despair." --Macaulay.
   [1913 Webster]

         A certain apathy or sluggishness in his nature which
         led him . . . to leave events to take their own course.
                                                  --Prescott.
   [1913 Webster]

         According to the Stoics, apathy meant the extinction of
         the passions by the ascendency of reason. --Fleming.
   [1913 Webster]

   Note: In the first ages of the church, the Christians adopted
         the term to express a contempt of earthly concerns.
         [1913 Webster]

   Syn: Insensibility; unfeelingness; indifference; unconcern;
        stoicism; supineness; sluggishness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

apathy
    n 1: an absence of emotion or enthusiasm
    2: the trait of lacking enthusiasm for or interest in things
       generally [syn: apathy, indifference, numbness,
       spiritlessness]

Moby Thesaurus II by Grady Ward, 1.0:

174 Moby Thesaurus words for "apathy":
   Laodiceanism, abeyance, abstraction, abulia, accidia, acedia,
   alienation, aloofness, anxiety, anxiety equivalent, anxiety state,
   ataraxia, ataraxy, benumbedness, blah, blahs, boredom, callousness,
   calmness, carelessness, casualness, catalepsy, catatonia,
   catatonic stupor, cave of Trophonius, cave of despair, coldness,
   comatoseness, compulsion, deadliness, deathliness, dejection,
   depression, despair, desperateness, desperation, despondency,
   detachment, disconsolateness, disinterest, disinterestedness,
   dispassion, dispassionateness, disregard, disregardfulness,
   dormancy, drowsiness, dullness, easygoingness, elation,
   emotionalism, enervation, ennui, entropy, euphoria, fatigue,
   folie du doute, forlornness, halfheartedness, hardness,
   heartlessness, heaviness, hebetude, heedlessness, hopelessness,
   hypochondria, hysteria, hysterics, impassiveness, impassivity,
   inanimation, inappetence, inattention, incuriosity, incuriousness,
   indifference, indifferentism, indifferentness, indiscrimination,
   indolence, inertia, inertness, inexcitability, insensibility,
   insensitivity, insouciance, intellectual inertia, jadedness,
   lack of affect, lack of appetite, lack of interest,
   lackadaisicalness, languidness, languishment, languor,
   languorousness, lassitude, latency, lenitude, lentor,
   lethargicalness, lethargy, lifelessness, listlessness,
   lotus-eating, lukewarmness, mania, melancholia, mental distress,
   mindlessness, negligence, no exit, no way, no way out, nonchalance,
   numbness, obduracy, obsession, oscitancy, passiveness, passivity,
   pathological indecisiveness, phlegm, phlegmaticalness,
   phlegmaticness, plucklessness, pococurantism, preoccupation,
   psychalgia, psychomotor disturbance, recklessness, regardlessness,
   resignation, resignedness, satedness, sleepiness, sloth,
   slothfulness, slowness, sluggishness, somnolence, sopor,
   soporifousness, spiritlessness, spunklessness, stagnancy,
   stagnation, stasis, stoicism, stolidity, stupefaction, stupor,
   supineness, suspense, tic, torpidity, torpidness, torpitude,
   torpor, twitching, unanxiousness, unawareness, unconcern,
   uninquisitiveness, uninterestedness, unmindfulness,
   unresponsiveness, unsolicitousness, vegetation, vis inertiae,
   weariness, withdrawal, withdrawnness, world-weariness