Jeju is the name of an island, province, and city in South Korea. The island and province are both called Jeju-do (제주도; 濟州島 (island); 濟州道 (province)), and the city - the provincial capital - is called Jeju-si (제주시; 濟州市) in Korean.
Jeju-do - formerly known to Europeans as Quelpart - is a subtropical volcanic island, dominated by Halla-san, a volcano that is with 1,950m the tallest mountain in South Korea. Jeju-do became a province of South Korea in 1946 when it was separated from South Jeolla Province. The province is located in the Korean Strait (part of East China Sea) southwest of South Jeolla, and is the large island in the lower lefthand corner of the map shown here on the right.
Jeju's sister provinces are also islands: Hainan Province (China), Hawaii (US), Sakhalin (Russia), and Bali (Indonesia).