The term plenipotentiary (from the Latin, plenus + potens, full + power) refers to a person who has "full powers." In particular, the term is commonly used to refer to a diplomat or ambassador who is fully authorised to represent his government.

Historically, before systems of rapid international transport (such as cars, trains and aircraft) or virtually instantaneous communication (such as radio or telephone) were commonplace, ambassadors were granted full (plenipotentiary) powers to represent their government in negotiations with their host nation, in the expectation that any representations made or agreements reached with the ambassador would be recognised and complied with by the ambassadors's government. In modern times, heads of state and of government, and more junior ministers and officials, can easily meet or speak with each other personally, so ambassadors arguably do not require plenipotentiary powers, as they continue to be designated and accredited as extraordinary and plenipotentiary.

This word has been voted as one of the ten English words that are hardest to translate in June 2004 by Today Translations, a British translation company [1].