Kos or Cos (/kɒs, kɔːs/; Greek: Κως [kos]) is a Greek island, part of the Dodecanese island chain in the southeastern Aegean Sea. Kos is the third largest island of the Dodecanese by area, after Rhodes and Karpathos; it has a population of 33,388 (2011 census), making it the second most populous of the Dodecanese, after Rhodes. The island measures 40 by 8 kilometres (25 by 5 miles). Administratively, Kos constitutes a municipality within the Kos regional unit, which is part of the South Aegean region. The principal town of the island and seat of the municipality is Kos Town.
The name Kos (Ancient Greek: Κῶς, genitive Κῶ) is first attested in the Iliad, and has been in continuous use since. Other ancient names include Meropis, Cea, and Nymphaea.
In many Romance languages, Kos was formerly known as Stancho, Stanchio, or Stinco, and in Ottoman and modern Turkish it is known as İstanköy, all from the reinterpretation of the Greek expression εις την Κω 'to Kos'; cf. the similar Istanbul and Stimpoli, Crete. Under the rule of the Knights Hospitaller of Rhodes, it was known as Lango or Langò, presumably because of its length. In The Travels of Sir John Mandeville, the author misunderstands this and treats Lango and Kos as distinct islands.
In Italian, the island is known as Coo.
A person from Kos is called a "Koan" in English. The word is also an adjective, as in "Koan goods".
Kos is in the Aegean Sea. Its coastline is 112 kilometres (70 miles) long and it extends from west to east.
The island has several promontories, some with names known in antiquity: Cape Skandari, anciently Scandarium or Skandarion in the northeast; Cape Lacter or Lakter in the south; and Cape Drecanum or Drekanon in the west.
In addition to the main town and port, also called Kos, the main villages of Kos island are Kardamena, Kefalos, Tingaki, Antimachia, Mastihari, Marmari and Pyli. Smaller ones are Zia, Zipari, Platani, Lagoudi and Asfendiou.