The Collaborative International Dictionary of English v.0.48:

Carelessness \Care"less*ness\, n.
   The quality or state of being careless; heedlessness;
   negligence; inattention.
   [1913 Webster]

WordNet (r) 3.0 (2006):

carelessness
    n 1: the quality of not being careful or taking pains [syn:
         carelessness, sloppiness] [ant: carefulness]
    2: failure to act with the prudence that a reasonable person
       would exercise under the same circumstances [syn:
       negligence, carelessness, neglect, nonperformance]

Moby Thesaurus II by Grady Ward, 1.0:

160 Moby Thesaurus words for "carelessness":
   abandon, absentmindedness, accidia, acedia, aloofness, apathy,
   ataraxia, ataraxy, blowziness, blunderheadedness, blundering,
   boggling, boredom, botchery, botching, bungling, caprice,
   careless abandon, casualness, chintziness, cursoriness,
   desperateness, detachment, devil-may-careness, disinterest,
   disinterestedness, dispassion, disregard, disregardfulness,
   distraction, dowdiness, easiness, easygoingness, flightiness,
   flippancy, forgetfulness, frivolousness, frowziness, frumpishness,
   fumbling, furiousness, giddiness, grubbiness, haste, hastiness,
   heedlessness, hotheadedness, hurriedness, impassiveness,
   impassivity, impetuosity, impetuousness, impotence, imprecision,
   inadvertence, inadvertency, inattention, inattentiveness,
   inconsiderateness, inconsideration, incuriosity, incuriousness,
   indifference, indifferentism, indifferentness, indiscrimination,
   inexcitability, inobservance, insouciance, intellectual inertia,
   lack of affect, lack of foresight, lack of interest,
   lackadaisicalness, laxity, laxness, laziness, leniency, levity,
   lightmindedness, listlessness, looseness, loosening, messiness,
   mindlessness, muffing, negligence, nonchalance, nonobservance,
   oblivion, obliviousness, offhandedness, overeagerness,
   overenthusiasm, overindulgence, overpermissiveness,
   overzealousness, perfunctoriness, permissiveness, pococurantism,
   precipitance, precipitancy, precipitateness, precipitation,
   precipitousness, recklessness, regardlessness, relaxation,
   relaxedness, remissness, seediness, shabbiness, shallowness,
   shoddiness, slackness, slatternliness, slipshodness, sloppiness,
   sloth, slovenliness, slovenry, sluttishness, softness, sordidness,
   sprezzatura, squalidness, squalor, stolidity, superficiality,
   tackiness, tactlessness, tawdriness, thoughtlessness,
   too many cooks, unalertness, unanxiousness, unawareness, unconcern,
   unconsciousness, unheedfulness, uninquisitiveness, unintentiveness,
   uninterestedness, unmindfulness, unneatness, unobservance,
   unpreparedness, unreadiness, unrestraint, unsolicitousness,
   unsolicitude, unthinkingness, unthoughtfulness, untidiness,
   unwariness, unwatchfulness, wantonness, weakness, wildness,
   withdrawal