credential

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun That which entitles one to confidence, credit, or authority.
• noun Evidence or testimonials concerning one's right to credit, confidence, or authority.
• transitive verb To supply with credentials.

from The Century Dictionary.

• Giving a title to credit or confidence.
• noun That which gives credit; that which gives a title or claim to confidence.
• noun plural Evidences of right to credence or authority; specifically, letters of credence; testimonials given to a person as the warrant on which belief, credit, or authority is claimed for him, as the letters of commendation and authorization given by a government to an ambassador or envoy, which procure for him recognition and credit at a foreign court, or the certificate and other papers showing the appointment or election of an officer.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Giving a title or claim to credit or confidence; accrediting.
• noun That which gives a title to credit or confidence.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective of, pertaining to or entitling to credit or authority
• noun documentary evidence that a person has certain status or privileges
• verb to furnish with credentials

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a document attesting to the truth of certain stated facts