Source : Webster's Revised Unabridged Dictionary (1913)

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

         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.

Source : WordNet®

inattention
     n : lack of attention [ant: {attention}]