diploma vs credential

diploma and credential both are nouns.

Word	Noun	Adjective	Verb	Adverb
diploma	Yes	No	No	No
credential	Yes	No	No	No

As nouns, credential is a hypernym of diploma; that is, credential is a word with a broader meaning than diploma:

diploma: a document certifying the successful completion of a course of study
credential: a document attesting to the truth of certain stated facts

Other hypernyms of diploma include certificate, certification, credentials.

diploma (noun)	credential (noun)
a document certifying the successful completion of a course of study	a document attesting to the truth of certain stated facts