South Korea, officially the Republic of Korea, and commonly referred to as Korea, is a country in East Asia, constituting the southern part of the Korean Peninsula. The name Korea is derived from the Kingdom of Goryeo, also spelt as Koryŏ. It shares land borders with North Korea to the north, and oversea borders with Taiwan to the south, Japan to the east and China to the west. South Korea lies in the north temperate zone with a predominantly mountainous terrain. Roughly half of the country's 50 million people reside in the metropolitan area surrounding its capital, the Seoul Capital Area, which is the second largest in the world with over 25 million residents.