South Korea, officially the Republic of Korea, is a sovereign state located in the southern part of the Korean Peninsula. The name "Korea" is derived from Goryeo, a dynasty which ruled in the Middle Ages. Its neighbors are China to the west, Japan to the east, and North Korea to the north. South Korea lies in the north temperate zone with a predominantly mountainous terrain. It covers a total area of 99,392 square kilometers and has a population of 50 million.