According to the 1969 Vienna Convention on the Law of Treaties“Treaty” means an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation”.

A treaty may also be known as an (international) agreement, protocol, covenant, convention or exchange of letters, among other terms. Regardless of terminology, all of these forms considered treaties.

The 1986 Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations states that a treaty must be in a written form, must be governed by international law, the name (designation) of the agreement is not relevant, the form may be that of one or more related documents.

Source: Wikipedia