South Korea,[b] officially the Republic of Korea (ROK),[c] is a country in East Asia, constituting the southern part of the Korean Peninsula and sharing a land border with North Korea. Its western border is formed by the Yellow Sea, while its eastern border is defined by the Sea of Japan. South Korea has a population of 51 million, of which roughly half live in the Seoul Capital Area, the fifth largest metropolis in the world. Other major cities include Incheon, Busan, and Daegu.