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]