Letter of credence
A letter of credence (French: lettre de créance, [lɛtʁ də kʁeɑ̃s]) is a formal diplomatic letter that designates a diplomat as ambassador to another sovereign state. Commonly known as diplomatic credentials, the letter is addressed from one head of state to another, asking them to give "credence" (French: créance) to the ambassador's claim of speaking for their country.