What is Jus Tractatuum?

Legal Definition
Jus tractatuum (or sometimes jus tractandi) is a Legal Latin term commonly used in public international law and constitutional law that refers to the right to conclude treaties. It is usually referred to in English as treaty-making power. As defined in article 6 of the Vienna Convention on the Law of Treaties, every state possesses the capacity to conclude treaties. International organizations as well as subnational entities of federal states may have treaty-making power as well. Jus tractatuum is linked to the concept of international legal personality.
-- Wikipedia