Nanjing means "southern capital" literally. In many Chinese dynasties including the [six dynasties]? from 3rd to 6th century, (namely Wu2? (吳), [Dong1 Jin4]? (東晉), Song4? (宋), Qi2? (齊), Liang2? (梁), Chen2? (陳)), Nanjing was the capital city under various names. In middle Chinese, Nanjing was called Bo2 Xia4 (白下).
Compare to Beijing.