What does “Treaty” mean in International Law?
While it is true that defining the term treaty in International Law, in all its aspects, far exceeds the content of the following lines, I will try to express a general idea about the term.
Treaty and not “international treaty” (an expression that is clearly redundant) is a generic term that covers all binding instruments under international law, regardless of their formal designation, held between two or more entities with permanent or quota international legal personality.
Therefore, treaties can be concluded between: States; between International Organizations with capacity to enter into treaties; or between a mixture of the two previous subjects, that is, between States and International Organizations.
The use of the term treaty, in the generic sense, means that the parties intend to create demandable rights and obligations under international law, which is why saying that a treaty is an “international treaty” is redundant since, by definition, a treaty is an agreement made under the international law.
The 1967 Vienna Convention defines a treaty as “an international agreement concluded between States in writing and governed by international law, whether incorporated in a single instrument or in two or more related instruments and whatever their particular designation is” (Article 2 (1)).
In fact, as Matthias Herdegen points out in his book “Public International Law”:
For a long time, the rules on the entry into force and the legal effects of treaties were only in customary international law. The principle that international law treaties must be fulfilled (pacta sunt servanda) also finds its roots in customary international law.
The Vienna Convention on the 1969 Treaties Law codifies the existing rules of customary international law – many of which become part of this code by virtue of its effectiveness and reiterated practice by international parties.
International jurists have classified treaties according to a variety of principles. Besides the difference between multilateral and bilateral treaties, there is also the difference between treaties that represent a definite transaction (for example, a transfer of territory) and those that seek to establish a general rule of conduct (for example, “waiver to war”).
Treaties have also been classified according to their purpose, as follows:
(1) political treaties, including peace treaties, alliances, territorial transfers and disarmament treaties;
(2) commercial treaties, including tariff, consular, fisheries and navigation agreements;
(3) constitutional and administrative treaties, such as conventions that establish and regulate international unions, specialized organizations, and agencies;
(4) treaties related to criminal justice, such as treaties that define international crimes and that consider extradition;
(5) treaties related to civil justice, such as conventions for the protection of human rights, trademarks and copyright, and for the judgments execution of foreign courts; and
(6) treaties that codify international law, such as procedures for the peaceful settlement of international disputes, rules for war conduct and definitions of the states’ rights and duties.
Notwithstanding the foregoing classification, in practice, it is often difficult to assign a particular treaty to one of these categories, and the legal value of such distinctions is minimal.
 Herdegen, Matthias. (2017). Derecho internacional público. México: Instituto de Investigaciones Jurídicas de la UNAM. Pp. 117-118