(From Greek: diplous, "double" - that is, folded and sealed - via Latin)
Diplomas are letters which are imbued with an inherent legal validity. The term diploma originally described the military discharge notices handed over to Roman legionaries, but later came to apply to any letter of legal significance, whether issued by official or private sources.
Other words for diplomas are charter (Latin: carta, used to describe documents which are themselves institutive of law) and notice (Latin: notitia, used to describe a document which announces a legal decision not previously written down, i.e. an oral tradition or a decree).
The scientific study of diplomas is termed diplomatics.