South Korea, officially the Republic of Korea, is a sovereign state in the southern part of the Korean Peninsula. The name "Korea" is derived from Goryeo, a dynasty which ruled in the Middle Ages (an initial "G" in Korean is pronounced like an English "K"). Its neighbors are China to the west, Japan to the east, North Korea to the north, and the Korea Strait to the south. South Korea lies in the north temperate zone with a predominantly mountainous terrain.