Credential, testimonial, recommendation, character, reference mean something presented by one person to another in proof that he is what or who he claims to be.
Credential (usually in the plural credentials) implies material evidence and especially a letter or document indicating that a person (occasionally a thing) is what he claims (or it seems) to be; the term was originally and is still used of the letter from the sovereign or head of one state to another carried by a new envoy or ambassador and formally presented to the sovereign or head of the state in which he is to serve.
The term is often used of a letter presented to show competency or to attest identity, or of statements made or acts performed that serve as proof of what is to follow.
Testimonial usually implies a written statement from a person competent to judge the character, qualifications, or merits of another and to testify to his fitness to hold or to fill an office or a position.
However, the word is often used as an equivalent of recommendation, a term which implies that the statement comes from one (as a former employer or teacher) who commends a person to the notice of a possible employer.
Character, which in this sense is used chiefly in Great Britain, is the designation given to a statement furnished by a former employer about the qualities and habits of a person as manifested while in his employ.
Although reference may imply no more than the giving of the name of a person from whom information regarding another may be obtained (as by a possible employer or landlord) it increasingly tends to be employed as a synonym of recommendation or character.