Several factors affect witnesses' credibility. A credible witness is "competent to give evidence, and is worthy of belief." Generally, a witness is deemed to be credible if they are recognized (or can be recognized) as a source of reliable information about someone, an event, or a phenomenon.
Credible seems here to be used in the way that people have employed it ever since it appeared in the English language in the fourteenth century: of something that is convincing or is capable of being believed. (It came from Latin credibilis, worthy of being believed, from the verb credere, to believe.)
In the United States, such a witness is "more than likely to be true based on his/her experience, knowledge, training and appearance of honesty and forthrightness...." Some factors for determining the credibility of testimony in U.S. courts include: (1) the witness had personal knowledge, (2) he or she was actually present at the scene, (3) the witness paid attention at the scene, and (4) he or she told the whole truth. The probative value of a credible witness is not a required element in any criminal case. However, credibility is always a factor in civil cases. The number of witnesses does not matter for credibility: "The question for the jury is not which side has more witnesses, but what testimony they believe." Only the "quality or power" of believability matters.
In Australian law, the reliability of every witness in a criminal case must be taken into account.
Contesting the credibility of so-called "expert" witnesses rose into more common practice in the 1860s and 1870s.
For example, in most common law jurisdictions, two or three witnesses must sign their names to the Will at the attestation clause below the testimonium clause that is executed by the testator. The exact number of witnesses depend on the state or local law. Under the English Statute of Wills of 1540, it was three witnesses. Under modern New York law, only two witnesses are required. In Canadian law, a credible witness to a Will "means a witness not incapacitated by mental inbecility (sic), interest or crime."
^United States v. Welsh, op. cite, 774 F.2d 670 (4th Cir. 1985), found at US Courts website, citing Webster's 3rd New International Dictionary, p. 532, and Burleson v. State, 131 Tex.Cr.R. 576, 100 S.W.2d 1019, 1020 (1936).
^"Separate Consideration of Charges - Single Defendant," Model jury charges for a criminal case, Queensland, p. 1, footnote 2, found at Queensland Courts Government website, citingMarkuleski  NSW CCA 290; cf.Doggett  HCA 46 ; M  QCA 458 -; S  QCA 167 , . Accessed October 26, 2009.
^"Law and Police". Otago Daily Times. 18 September 1865. Retrieved 30 October 2011. "A strong effort was made to impeach her credibility as a witness... it is competent to prove that the witness is an expert and not a mere pretender."
^"Last Day of the Scandal Trial". Ithaca Democrat. 8 July 1875. Retrieved 30 October 2011. "There was an irreconcilable difference of opinion as to the credibility of witnesses on each side."