The Collaborative International Dictionary of English v.0.48:

Credential \Cre*den"tial\ (kr[-e]*d[e^]n"shal), a. [Cf. It.
   credenziale, fr. LL. credentia. See Credence.]
   Giving a title or claim to credit or confidence; accrediting.
   [1913 Webster]

         Their credential letters on both sides.  --Camden.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Credential \Cre*den"tial\, n. [Cf. It. credenziale.]
   1. That which gives a title to credit or confidence.
      [1913 Webster]

   2. pl. Testimonials showing that a person is entitled to
      credit, or has right to exercise official power, as the
      letters given by a government to an ambassador or envoy,
      or a certificate that one is a duly elected delegate.
      [1913 Webster]

            The committee of estates excepted against the
            credentials of the English commissioners.
                                                  --Whitelocke.
      [1913 Webster]

            Had they not shown undoubted credentials from the
            Divine Person who sent them on such a message.
                                                  --Addison.

WordNet (r) 3.0 (2006):

credential
    n 1: a document attesting to the truth of certain stated facts
         [syn: certificate, certification, credential,
         credentials]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "credential":
   advocacy, advocating, advocation, affidavit, attestation,
   authority, authorization, bill of health, certificate,
   certificate of character, certificate of proficiency,
   certification, character, character reference, deposition, diploma,
   letter of introduction, navicert, notarized statement, note,
   patronage, recommend, recommendation, reference, sheepskin,
   sworn statement, testamur, testimonial, ticket, visa, vise,
   voucher, warrant, warranty, witness