Source : Webster's Revised Unabridged Dictionary (1913)

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

         Of every species of incredulity, religious unbelief is
         the most irrational.                     --Buckminster.

Source : WordNet®

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