What is Credential?

Legal Definition
A credential is an attestation of qualification, competence, or authority issued to an individual by a third party with a relevant or de facto authority or assumed competence to do so.

Examples of credentials include academic diplomas, academic degrees, certifications, security clearances, identification documents, badges, passwords, user names, keys, powers of attorney, and so on. Sometimes publications, such as scientific papers or books, may be viewed as similar to credentials by some people, especially if the publication was peer reviewed or made in a well-known journal or reputable publisher.
-- Wikipedia
Legal Definition
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, 76.
-- Bouviers Law Dictionary
Legal Definition
In 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, ยง 76.
-- Black's Law Dictionary