The Collaborative International Dictionary of English v.0.48:

Diploma \Di*plo"ma\, n.; pl. Diplomas. [L., fr. Gr. ?, fr. ?
   to double, fr. diplo`os twofold. See Double.]
   A letter or writing, usually under seal, conferring some
   privilege, honor, or power; a document bearing record of a
   degree conferred by a literary society or educational
   institution.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diploma
    n 1: a document certifying the successful completion of a course
         of study [syn: diploma, sheepskin]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "diploma":
   affidavit, attestation, authority, authorization, bill of health,
   brevet, certificate, certificate of proficiency, certification,
   charter, concession, credential, deposition, franchise, grant,
   letters patent, liberty, navicert, notarized statement, note,
   patent, royal grant, sheepskin, sworn statement, testamur,
   testimonial, ticket, visa, vise, voucher, warrant, warranty,
   witness



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DIPLOMA. An instrument of writing, executed by, a corporation or society,
certifying that a certain person therein named is entitled to a certain
distinction therein mentioned.
     2. It is usually, granted by learned institutions to their members, or
to persons who have studied in them.
     3. Proof of the seal of a medical institution and of the signatures of
its officers thereto affixed, by comparison with the seal and signatures
attached to a diploma received by the witness from the same institution, has
been held to be competent evidence of the genuineness of the instrument,
although the witness never saw the officers write their names. 25 Wend. R.
469.
     4. This word, which is also written duploma, in the civil law,
signifies letters issued by a prince. They are so called, it is supposed, a
duplicatis tabellis, to which Ovid is thought to allude, 1 Amor. 12, 2, 27,
when he says, Tunc ego vos duplices rebus pro nomine sensi Sueton in
Augustum, c. 26. Seals also were called Diplomata. Vicat ad verb.