“Treaty” is a very misunderstood term, so we need a definition of treaty that is precise and authoritative. The definition that makes reference is the one found in the Vienna Convention on the Law of Treaties of 1969:
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.
Now, there is also a Vienna Convention of 1986 which deals with agreement between States and international organizations, but for the primary definition deals with agreement between States. The requirements are:
– the agreement must be in writing
– the agreement must be governed by international law, which means that it creates laws and obligations under international law
– the name (designation) of the agreement is not relevant (an “Exchange of Notes” may well be a treaty)
– the form may be that of one or more related documents, for example an exchange of letters (two documents) can form a single treaty.