I think the word you're looking for is "authenticate." To “notarize” a document means you sign it in front of a notary public. A notary public is someone who is appointed by the government to serve as a witness when documents are signed. In my experience, they do not look at the document itself; they just verify that the signature is genuine. The notary public will check your picture ID to confirm your identity and watch you as you sign the document. The notary public will stamp an official notary seal by your signature that has information such as the notary public’s name and number. Each state has its own notary seal. This is the common definition of notarize in the US. I think what Gary is talking about is a “certified copy” in the US. Notary publics can certify documents too. A notary public is authorized to perform various functions, such as take oaths. To “accredit” means an official organization gives someone or something recognition that it has passed a certain standard. For example, an “accredited investor” or an “accredited law school.” I don’t think a document can be accredited in the conventional sense. To answer your question, since you seem to be talking about a diploma, I’m guessing the word you’re actually looking for is “authenticate.” I’ve had my US diploma authenticated at a Taiwanese consulate in the US so I can use it in Taiwan. In my case, they also made certified copies and authenticated those for me too.

Rue was quite correct about who and what gets accredited/notarised. Accrediting is a process by which people or organisations are allowed or authorised to carry out some function. Notarising gives some validity to a document. Usually it means that a respected (or possibly accredited! ;) person has seen an original document, and they will notarise a copy of that document, to say that the copy is a true and correct copy of the original. That way you don't have to send your valuable original document away with (say) some application.

People like diplomats or journalists and institutions like universities are accredited. Documents are notarized.