South Korea, officially the Republic of Korea , is a country in East Asia. It constitutes the southern part of the Korean Peninsula and borders North Korea along the Korean Demilitarized Zone. The country's western border is formed by the Yellow Sea, while its eastern border is defined by the Sea of Japan. South Korea claims to be the sole legitimate government of the entire peninsula and adjacent islands. It has a population of 51.96 million, of which roughly half live in the Seoul Capital Area, the fourth most populous metropolitan area in the world. Other major cities include Incheon, Busan, and Daegu.