Passage 7.1.1
ἡ δὲ τῆς Ἠλείας μέση καὶ Σικυωνίας, καθήκουσα μὲν ἐπὶ τὴν πρὸς ἕω θάλασσαν, Ἀχαΐαν δὲ ὄνομα τὸ ἐφʼ ἡμῶν ἔχουσα ἀπὸ τῶν ἐνοικούντων, αὐτή τε Αἰγιαλὸς τὸ ἀρχαῖον καὶ οἱ νεμόμενοι τὴν γῆν ἐκαλοῦντο Αἰγιαλεῖς, λόγῳ μὲν τῷ Σικυωνίων ἀπὸ Αἰγιαλέως βασιλεύσαντος ἐν τῇ νῦν Σικυωνίᾳ, εἰσὶ δὲ οἵ φασιν ἀπὸ τῆς χώρας, εἶναι γὰρ τὰ πολλὰ αὐτῆς αἰγιαλόν.
The region situated between Elis and Sicyonia, stretching down towards the eastern sea and known in our time as Achaia from its current inhabitants, was in ancient times called Aigialos, and those who lived there were called Aigialeis. According to the Sicyonian tradition, this name is derived from Aigialeus, who once ruled the territory now known as Sicyonia; but others claim the name comes from the nature of the country itself, since the greater part of it lies along the coast.