WordNet (r) 3.0 (2006):

credentials
    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:

30 Moby Thesaurus words for "credentials":
   ID card, badge, calligram, calling card, card, certification,
   character, countermark, countersign, documentation, documents,
   dog tag, endorsement, identification, identification badge,
   identification tag, initials, letter of introduction, monogram,
   papers, press card, recommendation, reference, sanction,
   serial number, signature, tessera, testimonial, visiting card,
   voucher



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

CREDENTIALS, international law. The instruments which authorize and
establish a public minister in his character with the state or prince to
whom they are addressed. If the state or prince receive the minister, he can
be received only in the quality attributed to him in his credentials. They
are, as it were, his letter of attorney, his mandate patent, mandatum
manifestum. Vattel, liv. 4, c. 6, Sec. 76.