Chios (ˈkaɪ.ɒs,_ˈkaɪ.oʊs,_ˈkiː-; Chíos ˈçi.os, traditionally known as Scio in English) is the fifth largest Greek island, situated in the northern Aegean Sea, and the tenth largest island in the overall Mediterranean Sea. The island is separated from Turkey by the Chios Strait. Chios is notable for its exports of mastic gum and its nickname is "the Mastic Island". Tourist attractions include its medieval villages and the 11th-century monastery of Nea Moni, a UNESCO World Heritage Site.
Administratively, the island forms a separate municipality within the Chios regional unit, which is part of the North Aegean region. The principal town of the island and seat of the municipality is Chios. Locals refer to Chios town as Chora (Χώρα literally means land or country, but usually refers to the capital or a settlement at the highest point of a Greek island).
The island was also the site of the Chios massacre, in which thousands of Greeks on the island were massacred, expelled, and enslaved by Ottoman troops during the Greek War of Independence in 1822. Chios remained a part of the Ottoman Empire until 1912.
Chios island is crescent or kidney-shaped, long from north to south, and at its widest, covering an area of . The terrain is mountainous and arid, with a ridge of mountains running the length of the island. The two largest of these mountains, Pelineon () and Epos (), are situated in the north of the island. The center of the island is divided between east and west by a range of smaller peaks, known as Provatas.
Chios can be divided into five regions:
Midway up the east coast lie the main population centers, the main town of Chios, and the regions of Vrontados and Kambos. Chios Town, with a population of 32,400, is built around the island's main harbour and medieval castle. The current castle, with a perimeter of , was principally constructed during the time of Genoese and Ottoman rule, although remains have been found dating settlements there back to 2000 B.C. The town was substantially damaged by an earthquake in 1881, and only partially retains its original character.