The Collaborative International Dictionary of English v.0.48:

Incredulity \In`cre*du"li*ty\, n. [L. incredulitas: cf. F.
   incr['e]dulit['e].]
   The state or quality of being incredulous; a withholding or
   refusal of belief; skepticism; unbelief; disbelief.
   [1913 Webster]

         Of every species of incredulity, religious unbelief is
         the most irrational.                     --Buckminster.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incredulity
    n 1: doubt about the truth of something [syn: incredulity,
         disbelief, skepticism, mental rejection]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "incredulity":
   Humism, Pyrrhonism, agnosticism, atheism, denial, disbelief,
   discredit, doubt, heresy, inability to believe, infidelity,
   minimifidianism, misbelief, nonbelief, nullifidianism, rejection,
   scoffing, skepticism, unbelief, unbelievingness, unfaith