The Collaborative International Dictionary of English v.0.48:

Believable \Be*liev"a*ble\, a.
   Capable of being believed; credible. -- Be*liev"a*ble*ness,
   n. -- Be*liev`a*bil"i*ty, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

believability
    n 1: the quality of being believable or trustworthy [syn:
         credibility, credibleness, believability] [ant:
         incredibility, incredibleness]