Achaia, the ancient name of a country in the Peloponnesus, lying along the S. coast of the Corinthian Gulf, also called AEgialea. It was peopled by the Achaians, who originally came from Thessaly and conquered the greater part of the Peloponnesus; but on the return of the Heraclidas they were driven to the N. coast. There the name spread very widely; and about 280 B.C. was formed a confederacy, which embraced twelve cities, known as the Achaian League. Under Aratus and Philopoemen this remarkable organisation kept alive the traditions of independence, and afforded a model of federal government. When the Romans conquered Greece, they gave the name of Achaia to the southern portion of the country, formerly known as the Peloponnesus. With Macedonia it constituted the whole of Greece, and consequently the phrase Macedonia and Achaia came to be used as an equivalent for the ancient Greece. It now forms, together with Elis, a province which occupies much the same situation as the ancient Achaia.