The Collaborative International Dictionary of English v.0.48:

acedia \acedia\ n.
   1. apathy and inactivity in the practice of virtue
      (personified as one of the deadly sins).

   Syn: sloth, laziness
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

acedia
    n 1: apathy and inactivity in the practice of virtue
         (personified as one of the deadly sins) [syn: sloth,
         laziness, acedia]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "acedia":
   accidia, aloofness, anger, apathy, ataraxia, ataraxy, avarice,
   avaritia, benumbedness, blah, blahs, boredom, carelessness,
   casualness, cave of Trophonius, cave of despair, comatoseness,
   deadly sin, despair, desperateness, desperation, despondency,
   detachment, disconsolateness, disinterest, dispassion, disregard,
   disregardfulness, drowsiness, dullness, easygoingness, enervation,
   ennui, envy, fatigue, forlornness, gluttony, greed, gula,
   heartlessness, heaviness, hebetude, heedlessness, hopelessness,
   inanimation, inappetence, inattention, incuriosity, indifference,
   indiscrimination, inexcitability, insouciance, invidia, ira,
   jadedness, lack of affect, lack of appetite, lackadaisicalness,
   languidness, languishment, languor, languorousness, lassitude,
   lenitude, lentor, lethargicalness, lethargy, lifelessness,
   listlessness, lust, luxuria, mindlessness, negligence, no exit,
   no way, no way out, nonchalance, numbness, oscitancy, passiveness,
   passivity, phlegm, phlegmaticalness, phlegmaticness, plucklessness,
   pococurantism, pride, recklessness, regardlessness, resignation,
   resignedness, satedness, sleepiness, sloth, slothfulness, slowness,
   sluggishness, somnolence, sopor, soporifousness, spiritlessness,
   spunklessness, stupefaction, stupor, superbia, supineness,
   torpidity, torpidness, torpitude, torpor, unanxiousness, unconcern,
   unmindfulness, unsolicitousness, weariness, withdrawnness,
   world-weariness, wrath