Source : Webster's Revised Unabridged Dictionary (1913)

Believable \Be*liev"a*ble\, a.
   Capable of being believed; credible. -- {Be*liev"a*ble*ness},
   n. -- {Be*liev`a*bil"i*ty}, n.

Source : WordNet®

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