Ionia (aɪˈoʊniə) was an ancient region on the western coast of Anatolia, to the south of present-day Izmir, Turkey. It consisted of the northernmost territories of the Ionian League of Greek settlements. Never a unified state, it was named after the Ionians who had settled in the region before the Archaic period.
Ionia proper comprised a narrow coastal strip from Phocaea in the north near the mouth of the river Hermus (now the Gediz), to Miletus in the south near the mouth of the river Maeander, and included the islands of Chios and Samos. It was bounded by Aeolia to the north, Lydia to the east and Caria to the south. The cities within the region figured significantly in the strife between the Persian Empire and the Greeks.
Ionian cities were identified by mythic traditions of kinship and by their use of the Ionic dialect, but there was a core group of twelve Ionian cities that formed the Ionian League and had a shared sanctuary and festival at Panionion. These twelve cities were (from south to north): Miletus, Myus, Priene, Ephesus, Colophon, Lebedos, Teos, Erythrae, Clazomenae and Phocaea, together with the islands of Samos and Chios. Smyrna, originally an Aeolic colony, was afterwards occupied by Ionians from Colophon, and became an Ionian city.
The Ionian school of philosophy, centered on 6th century BC Miletus, was characterized by a focus on non-supernatural explanations for natural phenomena and a search for rational explanations of the universe, thereby laying the foundation for scientific inquiry and rational thought in Western philosophy.
Ionia was of small extent, not exceeding in length from north to south, with the cities located on a narrow band between the sea and the mountains, which varies in width from . So intricate is the coastline that the voyage along its shores was estimated at nearly four times the direct distance. The location of the eastern border with Lydia and Caria was vague in antiquity.