koinon

Koinon (Greek: κοινόν) is a neuter noun meaning “the common thing” or “the public.” In classical and Hellenistic Greek usage it referred to a political or social entity formed by multiple communities sharing a common interest, institution, or treasury. The word is related to the adjective koinos “common” and to koine, the term later used for the common or shared language of the Hellenistic world.

In political vocabulary, koinon describes leagues or federations of city-states—a commonwealth or league with its own institutions and finance, sometimes featuring a central treasury and an assembly representing its members. Notable examples include the Delian koinon, the Achaean koinon, and the Boeotian koinon, among others. The concept emphasizes shared defense, governance, and resources rather than a single city-state’s sovereignty.

In other uses, koinon can denote the public or shared life of a community, such as common property, resources, or duties held in trust for the group. The term appears across ancient Greek political and literary texts as a way to discuss the balance between local autonomy and unified interests. The related noun koinonia (fellowship or communion) derives from the same root and reflects the broader idea of communal participation and shared life. In modern Greek, koinon can simply mean “public” or “common,” retaining its core sense of collective belonging.