This commonly refers to a diplomat that has the full signing power and authority of their government. They can agree to treaties with other nations.
This position/title was more important before instant communication. Before long-distance, real-time communication was possible, the foreign representatives of a government might be incommunicado for months at a time, so they needed some amount of authority and self-governance.
Why I Looked It Up
Found in a passage about premature news of the signing of the armistice that ended World War 1.
…a telegram reached Howard from the United Press office in Paris: “Armistice report untrue. War Ministry issues absolute denial and declares enemy plenipotentiaries to be still on way through lines.”
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