Korea, officially the Republic of Korea, is a sovereign state in 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, Heilong to the North, and the East China Sea to the south. Korea lies in the north temperate zone with a predominantly mountainous terrain. The Korean economy grew significantly and the country was transformed into a major economy, a full democracy, and a regional power in East Asia.