During the days of the Roman Republic and of the later empire a person upon whom certain rights or privileges were conferred received an official document, signed and sealed by the consuls and senate or, later, by the emperor or such magistrate as he might designate.
This document or state letter consisted of two leaves, either two tablets of wax or a folded sheet of writing material, and it was therefore known by the Greek name for such a twofold sheet, diploma, from diploos, double.
Public couriers especially, on errands to foreign cities or countries, carried these tokens of authority. Because a diploma was essentially a state document, the term carried that meaning among the scholars of Europe.
Therefore when a German writer of the seventeenth century undertook to compile a collection, in the original texts, of the important public documents between the eleventh and fifteenth centuries, he coined the Latin adjective diplomaticus to indicate the nature of his collection.
This provided us with the English adjective, diplomatic, which, though it first pertained to original official documents, came to refer to documents relating to international affairs, the usual nature of such official documents. From the adjective in its revised sense came the noun diplomacy, the art of conducting affairs between nations.
Then, but not until the early nineteenth century, the nations found it expedient to rely upon persons who possessed certain skills in handling foreign relations, and the term diplomat was created. The original notion of a state paper is retained by all these terms.
We are more accustomed to the use of diploma, however, as an indication of scholastic attainment; but here, too, it was originally a state paper, and is still issued under the license of the state.