Source : Webster's Revised Unabridged Dictionary (1913)

Carelessness \Care"less*ness\, n.
   The quality or state of being careless; heedlessness;
   negligenece; inattention.

Source : WordNet®

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