Abuja became Nigeria's capital in 1991, replacing Lagos. The purpose-built city sits near the country's geographic center and was selected to provide a more neutral and accessible federal capital.
Lagos had served as Nigeria's capital during the colonial period and after independence, but its rapid growth created serious congestion and space problems. Lagos is also located in the country's southwest, while Nigeria's many ethnic and regional communities made a central capital politically attractive. The decision to move the capital was announced in 1976.
Abuja was planned and developed as a new administrative city. Its master plan was prepared by the Japanese architect Kenzo Tange and his team, and construction proceeded through the late twentieth century. The city formally replaced Lagos as the seat of the federal government on 12 December 1991.
Lagos remains Nigeria's largest city and a major commercial, cultural, and financial center, so it is a common wrong answer in this question. Abuja, however, is the national capital and hosts institutions such as the National Assembly, the Supreme Court, and the presidential complex.