What Is an International Treaty? Definition, Synonyms and Types

An international treaty is a legally binding agreement made between subjects of international law, usually states or international organizations. Its purpose is to create rights and obligations under international law. Common treaty labels include agreement and convention. Other labels, such as charter, protocol or concordat, can also describe binding instruments. The name alone is not decisive; instead, the key question is whether the parties intended to be legally bound. Some documents, such as gentlemen’s agreements or many memoranda of understanding, may carry political weight without being treaties.

Definition of International Treaty

An international treaty is defined as a formal agreement established between subjects of international law aimed at producing legal effects. The definition separates three legal questions: whether the instrument is formal, who the parties are and what legal consequences it creates.

Treaties, by their nature, are formal agreements. The parties must explicitly express their will to be bound by the agreement. That expression gives all participants a shared understanding of the commitment undertaken. International custom usually requires a state’s will regarding treaty content to appear in writing. However, international law recognizes exceptions to this rule. Some treaties may not be written, such as oral treaties. While recognized by international entities like the UN International Law Commission, oral treaties remain exceptional.

Furthermore, only subjects of international law are empowered to enter into treaties. A subject of international law can hold rights and assume obligations within the international legal order. Treaty-making capacity is narrower than international legal personality. For example, individuals can be recognized in certain contexts within international law, but they generally cannot conclude treaties on their own.

Finally, an international treaty is an instrument intended to produce legal effects. In other words, its reason for being is to produce new norms — rights or obligations — within the international legal system. For an agreement to be considered a treaty, the parties must show an explicit intention to be legally bound. That intention is known as animus contrahendi, or the intent to contract. Animus contrahendi refers to the real will of the parties to assume obligations through the agreement.

Synonyms for International Treaty

In the realm of International Law, several terms refer to the legal instruments that States and other legal subjects use to regulate their relations. Although commonly grouped under the generic term “international treaty”, these terms have specific meanings and applications. Some terms reflect the nature or purpose of the agreement. Others describe documents that are not treaties in the technical legal sense.

Terms that often denote treaties include:

• Agreement: Generally used to refer to an act of lesser significance or that involves few participants. Its flexibility allows it to adapt to various international situations.
• Charter or Constitution: This term applies to the foundational acts of international organizations, outlining their structure and functions.
• Statute: Refers to the legal instrument that establishes and regulates international courts, defining their jurisdiction and procedures.
• Commitment: An act through which parties submit a dispute to arbitration, specifying the conditions and terms of the process.
• Concordat: Refers to treaties made between the Holy See and other parties on religious matters or the organization of the Church.
• Convention: Identifies a multilateral act intended to create norms of general application, often with numerous signatories.
• Agreement: Used for cooperation agreements on specific and diverse topics, which can be bilateral or multilateral.
• Modus Vivendi: Refers to a temporary arrangement aimed at maintaining the status quo or establishing bases for future negotiations.
• Pactum de Negotiando: An obligation to enter into negotiations to conclude a treaty on a specific matter.
• Pactum de Contrahendo: A firm commitment to conclude a final agreement on a certain matter.
• Protocol: Can refer to the proceedings of a conference or to the norms and decisions emanating from it.

Related terms that usually do not denote treaties include:

• Gentlemen’s Agreement: An informal understanding grounded in honor among statesmen. It may reflect serious intentions, but it does not create legal obligations under international law.

• Declaration: An act that enshrines principles, often of an ethical or political nature, without necessarily creating legally binding obligations.

• Memorandum of Understanding: Although concluded between subjects of international law, this legal instrument lacks the necessary animus contrahendi to be considered a treaty, since it primarily contains political exhortations without legal binding force.

The distinction between these terms affects how international law is interpreted and applied. The name and subject matter of an instrument are only starting points. Lawyers also examine its production process and final form. The decisive question is whether international subjects, mainly states, created the instrument with the intention of producing concrete legal effects.

Types of International Treaties

International treaties can be classified by their characteristics, objectives and scope. The main legal and political classifications are:

• Bilateral treaties involve only two parties. Multilateral treaties include three or more parties. The Charter of the United Nations is a major multilateral example and has been ratified by 193 countries.
• Open treaties allow other states or entities to join after conclusion. Closed treaties allow new parties only with authorization from all existing parties.
• Treaties with a short procedure require fewer formalities for entry into force, usually signature or endorsement by a representative. Treaties with a long procedure require more complex processes and often depend on ratification after parliamentary approval.
• Transitory treaties have an immediate effect and create a situation that persists over time, such as a boundary settlement. Permanent treaties extend compliance and effects over time, as trade treaties and human rights treaties usually do.
• Restricted effect treaties bind only the signing parties. Non-restricted effect treaties can influence other subjects of international law beyond the signatories.

The Vienna Conventions on Treaties

In international law, the Vienna Conventions of 1969 and 1986 are the two main treaty-law instruments. They set rules for defining and creating treaties. They also govern interpretation, implementation, modification and termination.

The Vienna Convention on the Law of Treaties of 1969 (VCLT/69) came into effect in 1980. It focuses exclusively on treaties concluded between states, providing a formal and comprehensive definition of what constitutes such a document. According to this convention, a treaty is “an international agreement concluded in writing between States and governed by International Law, whether embodied in a single instrument or in two or more related instruments and regardless of its particular designation”. According to Article 5 of the VCLT/69, it applies to any treaty that is an instrument constituting an international organization and to any treaty adopted within the framework of an international organization, without prejudice to any relevant rules of the organization.

The Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations of 1986 (VCLT/86) was intended to govern treaties involving international organizations. The convention reflects the growing treaty-making role of those organizations. It also addresses the need to regulate agreements in which they participate. As of June 2026, the UN Treaty Collection still lists the VCLT/86 as not yet in force because Article 85 requires 35 state ratifications or accessions, and international organizations that join the convention do not count toward that entry-into-force threshold. Nevertheless, many of its rules are treated as part of customary international law — that is, the body of unwritten international norms.

Conclusion

International treaties are central instruments for regulating relations between legal subjects at a global level. The diversity of treaty terminology shows why legal form cannot be inferred from a document’s title alone. Distinguishing treaties from related documents improves negotiation, implementation and dispute resolution over international rights and obligations. The Vienna Conventions of 1969 and 1986 provide the main international rules on treaty creation. They also govern modification, implementation and termination.