Persona non grata
In diplomacy, a persona non grata (Latin: "person not appreciated", plural: personae non gratae) is a foreign person whose entering or remaining in a particular country is prohibited by that country's government. Being so named is the most serious form of censure which a country can apply to foreign diplomats, who are otherwise protected by diplomatic immunity from arrest and other normal kinds of prosecution.
Under Article 9 of the Vienna Convention on Diplomatic Relations, a receiving state may "at any time and without having to explain its decision" declare any member of a diplomatic staff persona non grata. A person so declared is considered unacceptable and is usually recalled to his or her home nation. If not recalled, the receiving state "may refuse to recognize the person concerned as a member of the mission".
With the protection of mission staff from prosecution for violating civil and criminal laws, depending on rank, under Articles 41 and 42 of the Vienna Convention, they are bound to respect national laws and regulations. Breaches of these articles can lead to a persona non grata declaration being used to punish erring staff. It is also used to expel diplomats suspected of espionage (described as "activities incompatible with diplomatic status") or any overt criminal act such as drug trafficking. The declaration may also be a symbolic indication of displeasure.
So-called "tit for tat" exchanges have occurred (whereby ambassadors of countries involved in a dispute each expel the ambassador of the other country), notably during the Cold War. A notable occurrence outside of the Cold War was an exchange between the United States and Ecuador in 2011: the Ecuadorian government expelled the United States ambassador, as a result of diplomatic cables leaking (WikiLeaks), the United States responded by expelling the Ecuadorian ambassador.
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In non-diplomatic usage, referring to someone as persona non grata is to say that the person is ostracized. The person then is, for all intents and purposes, culturally shunned and is figuratively nonexistent.