What is Full Powers?

Legal Definition
Full Powers is a term in international law and is the authority of a person to sign a treaty or convention on behalf of a sovereign state. Persons other than the head of state, head of government or foreign minister of the state must produce Full Powers in order to sign a treaty binding their government. Such a person is called a plenipotentiary.

It is defined in articles 2 and 7 of the Vienna Convention on the Law of Treaties.
-- Wikipedia