Koinon (Greek: Κοινόν, pl. Κοινά, Koina), meaning "common," in the sense of "public," had many interpretations, some societal, some governmental. The word was the neuter form of the adjective, roughly equivalent in the governmental sense to Latin res publica, "the public thing." Among the most frequent uses is "commonwealth," the government of a single state, such as the Athenian. Frequent in the historical writings was a sense of "league" or "federation" an association of distinct city-states. As government of a league, Koinon comprised such functions as defense, diplomacy, economics, and religious practices among its member states. The word was carried over to other political associations in mediaeval and modern Greek history.
In Epirus itself there had in ancient times existed the Koinon of the Molossians. There was a Koinon of Laconians, centred on Sparta and its old dominions for a period under Roman rule, a Koinon of the Macedonians, also under Roman rule. In modern Greek history, during the Greek War of Independence, a local self-government termed Koinon was set up in the islands of Hydra, Spetsai and Psara.
Some federations termed Koinon were:
- Ionian League (Koinon Ionon), formed in the 7th century BC
- Koinon of the Aeinautae, recorded on an inscription which was found in Eretria, island Euboea, dated to the 5h century BC
- Chalcidian League (Koinon Chalkideon), existing ca. 430 BC to 348 BC
- Epirote League (Koinon Epiroton), existing from ca. 320 BC to ca. 170 BC
- Koinon of Crete under the Roman Empire to the 4th century
- Koinon of Macedonians existing from 3rd century to Roman period
- Lycian League, founded in 168 BC
- Koinon of Free Laconians, a league of cities in Laconia established by Roman emperor Augustus in 21 BC
- Koinon of the Zagorisians under the Ottoman Empire, 1670–1868