In diplomacy, a persona non grata (Latin: "person not welcome", plural: personae non gratae) is a status sometimes applied by a host country to foreign diplomats to remove their protection 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".
A person can be declared a persona non grata before that person even enters the country.
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 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.
People other than diplomats can be declared as persona non grata by a country.
In non-diplomatic usage, referring to someone as persona non grata is to say that the person is not popular or accepted by others.
The Soviet Foreign Ministry accused the five Americans of engaging in "actions which are incompatible with their official status"--the normal diplomatic jargon for espionage.
The Spanish foreign ministry spokesman said the two Russians had been expelled "for activities incompatible with their status as diplomats" - the diplomatic term for spying.
"Persona non grata" (PNG), is the most serious form of censure a government can take against a person with diplomatic immunity and often used by governments as symbolic expressions of displeasure.