An international alliance involving many different countries WordNet 3.0 & Open English WordNet

An international organization, also known as an intergovernmental organization or an international institution, is an organization that is established by a treaty, or is an instrument governed by international law and possessing its own legal personality, such as the United Nations, the World Health Organization, Save the Children International, and NATO. Wikipedia

Organization with an international membership, scope, or presence Wikidata

An organisation established by a treaty or other instrument governed by international law and possessing its own international legal personality. Wiktionary

Organisation established by treaty. Wiktionary (translation)