A diplomatic representative or official envoy, sometimes referring specifically to a lower-ranking diplomat.
From French 'démarche,' ultimately from de- + marcher (to march, to go). The word originally meant a course of action or proceeding, then narrowed to diplomatic context.
A demarch might deliver a formal protest from one country to another—it's one of those diplomatic customs that sounds quaint but is actually how nations avoid wars by having a protocol for anger.
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