The Collaborative International Dictionary of English v.0.48:

Inattention \In`at*ten"tion\, n. [Pref. in- not + attention: cf.
   F. inattention.]
   Lack of attention, or failure to pay attention; disregard;
   heedlessness; neglect.
   [1913 Webster]

         Novel lays attract our ravished ears;
         But old, the mind inattention hears.     --Pope.

   Syn: Inadvertence; heedlessness; negligence; carelessness;
        disregard; remissness; thoughtlessness; neglect.

   Usage: Inattention, Inadvertence. We miss seeing a thing
          through inadvertence when do not happen to look at it;
          through inattention when we give no heed to it, though
          directly before us. The latter is therefore the worse.
          Inadvertence may be an involuntary accident;
          inattention is culpable neglect. A versatile mind is
          often inadvertent; a careless or stupid one is
          inattentive.
          [1913 Webster]

WordNet (r) 3.0 (2006):

inattention
    n 1: lack of attention [ant: attending, attention]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "inattention":
   accidia, acedia, apathy, ataraxia, ataraxy, callowness,
   carelessness, casualness, childishness, culpa, culpable negligence,
   default, delinquency, dereliction, detachment, disconformity,
   disinterest, dispassion, disregard, disregardfulness,
   distractedness, distraction, diversion, divided attention,
   easygoingness, failure, heedlessness, immaturity, imprudence,
   inadvertence, inadvertency, inadvisability, inconsideration,
   incuriosity, indiscreetness, indiscretion, indiscrimination,
   inexcitability, inexpedience, injudiciousness, inobservance,
   insouciance, irrationality, laches, lack of affect,
   lackadaisicalness, laissez-faire, lapse, laxity, laxness,
   listlessness, looseness, mindlessness, neglect, neglectfulness,
   negligence, nonadherence, nonchalance, noncompliance,
   nonconformance, nonconformity, nonfeasance, nonfulfillment,
   noninterference, nonobservance, nonperformance, nonrestriction,
   omission, overlooking, oversight, permissiveness, pococurantism,
   pompousness, poor stewardship, procrastination, puerility,
   reasonlessness, recklessness, regardlessness, remissness,
   senselessness, slackness, slight, sloth, stuffiness,
   thoughtlessness, unanxiousness, unconcern, unintelligence,
   unmindfulness, unobservance, unreason, unreasonableness,
   unrigorousness, unsensibleness, unsolicitousness, unsoundness,
   unthoughtfulness, unwisdom, unwiseness, withdrawal of attention,
   witlessness