A parlimentaire or parlementaire is defined by the U.S. Department of Defense as "an agent employed by a commander of belligerent forces in the field to go in person within the enemy lines for the purpose of communicating or negotiating openly and directly with the enemy commander".

Even in war, the belligerents sometimes need to communicate, or negotiate. In the Middle Ages, heralds were used to deliver declarations of war and ultimata as a form of one-sided communication. But for two-sided communication, agents were needed who could also negotiate. These usually operated under a flag of truce and enjoyed temporary inviolability according to the customs and laws of war. Breaches of the customary protection of parlimentaires were deemed perfidy.

Later, these customs and protections were codified in international law. Articles 32-34 of the Hague Conventions (1907) state:

Contraventions of these articles constitute war crimes that may be prosecuted before the International Criminal Court.