Cephalonia or Kefalonia, formerly also known as Kefallinia or Kephallenia (Greek: Κεφαλονιά or Κεφαλλονιά) is the largest of the Ionian Islands in western Greece. It is also a separate regional unit of the Ionian Islands region, and the only municipality of the regional unit. The capital of Cephalonia is Argostoli.
The size of the island is about 781 sq km (300 sq mi), and the present population density is 55 people per sq km (140/sq mi). The town of Argostoli has one-third of the island's inhabitants. Lixouri is the second major settlement, and the two towns together account for almost two-thirds of the prefecture's population.
Cephalonia is in the heart of an earthquake zone, and dozens of minor or unrecorded tremors occur each year. In 1953, a massive earthquake almost destroyed all settlement on the island, leaving only Fiscardo in the north untouched. Among important natural features are the Melissani the Drogarati caves and the Koutavos Lagoon in Argostoli.