Achaea or Achaia was (and is) the northernmost region of the Peloponnese, occupying the coastal strip north of Arcadia. Its approximate boundaries were to the south the mountain range of Erymanthus, to the south-east the range of Cyllene, to the east Sicyon, and to the west the Larissos river. Apart from the plain around Dyme, to the west, Achaea was generally a mountainous region.
The name of Achaea has a slightly convoluted history. Homer uses the term Achaeans as a generic term for Greeks throughout the Iliad; conversely, a distinct region of Achaea is not mentioned. The region later known as Achaea is instead referred to as Aegialus. Both Herodotus and Pausanias recount the legend that the Achaean tribe was forced out of their lands in the Argolis by the Dorians, during the legendary Dorian invasion of the Peloponnese. Consequently, the Achaeans forced the Aegialians (now known as the Ionians) out of their land. The Ionians took temporary refuge in Athens, and Aegialus became known as Achaea. It was supposedly for this reason that the region known as Achaea in Classical Greece did not correspond to Homeric references.
Under the Romans, Achaea was a province covering much of central and southern Greece. This is the Achaea referenced in the New Testament (e.g., Acts 18:12 and 19:21; Romans 15:26 and 16:5). However, Pausanias, writing in the 2nd century AD, devotes one of the books of Description of Greece to the ancient region of Achaea, showing that the name, locally at least, still preserved the use from the Classical period. The name, Achaea, was later used in the crusader state, the Principality of Achaea (1205–1432), which comprised the whole Peloponnese, thus more closely following Roman use. The modern Greek prefecture of Achaea is largely based on the ancient region.