In diplomatic terms, what does 'persona non grata' refer to?

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 named a persona non grata is the most serious form of censure that a country can apply to foreign diplomats, who are otherwise protected by diplomatic immunity from arrest and other normal kinds of prosecution. Not only is it a negative reflection on the diplomat, but on their country of origin as well.

A country does not have to justify its removal of a diplomat from their country, and the receiving nation may further punish a diplomat by declaring the diplomat a non-member of their mission, thereby making them accountable for any crimes or infractions of the law when they were in that country.

During war times, some countries have applied persona non grata to remove spies from their country. The response was sometimes a retaliatory removal of the other's diplomat.