The Collaborative International Dictionary of English v.0.48:

Credibility \Cred`i*bil"i*ty\ (kr[e^]d`[i^]*b[i^]l"[i^]*t[y^]),
   n. [Cf. F. cr['e]dibilit['e].]
   The quality of being credible; credibleness; as, the
   credibility of facts; the credibility of witnesses.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "credibility":
   absolute credibility, acceptability, believability, believableness,
   conceivability, credit, plausibility, reliability, tenability,
   trustworthiness, truth, truth-loving, truth-speaking,
   truth-telling, truthfulness, veraciousness, veracity, veridicality,
   verity



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CREDIBILITY. Worthiness of belief. To entitle a witness to credibility, he
must be competent. Vide Competency.
     2. Human testimony can seldom acquire the certainty of demonstration.
Witnesses not unfrequently are mistaken or wish to deceive; the most that
can be expected is that moral certainty which arises from analogy. The
credibility which is attached to such testimony, arises. from the double
presumption that the witnesses have good sense and intelligence, and that
they are not mistaken nor deceived; they are further presumed to have
probity, and that they do not wish to deceive.
     3. To gain credibility, we must be assured, first, that the witness has
not been mistaken nor deceived. To be assured as far as possible on this
subject, it is proper to consider the nature and quality of the facts
proved; the quality and person of the witness; the testimony in itself; and
to compare it with the depositions of other witnesses on the subject, and
with known facts. Secondly, we must be satisfied that he does not wish to
deceive: there are strong assurances of this, when the witness is under
oath, is a man of integrity, and disinterested. Vide Arch. Civ. Pl. 444; 5
Com. Dig. 449; 8 Watts, R. 227; Competency.