The Collaborative International Dictionary of English v.0.48:

Conscientiousness \Con`sci*en"tious*ness\, n.
   The quality of being conscientious; a scrupulous regard to
   the dictates of conscience.
   [1913 Webster]

WordNet (r) 3.0 (2006):

conscientiousness
    n 1: the quality of being in accord with the dictates of
         conscience [ant: unconscientiousness]
    2: the trait of being painstaking and careful [syn:
       conscientiousness, painstakingness] [ant:
       unconscientiousness]