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):

believable
    adj 1: capable of being believed; "completely credible
           testimony"; "credible information" [syn: credible,
           believable] [ant: incredible, unbelievable]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "believable":
   colorable, conceivable, convincing, credible, creditable,
   fiduciary, impressive, likely, meaningful, persuasive, plausible,
   possible, presumable, probable, rational, reasonable, reliable,
   satisfying, solid, substantial, supposable, tenable, trustworthy,
   trusty, unexceptionable, unimpeachable, unquestionable,
   worthy of faith