Chosen (朝鮮) was the name of
Korea when it was part of the
Japanese Empire. It comes from the Korean term
Choson, which was used to refer to the state of Korea from the 1200's to the end of World War II (it is now only used when referring to
North Korea:
South Korea calls itself
Hanguk). Japan, it is said, supported the name "Korea" from 1905 to 1910 so that Japan and Korea would be next to each other on an
alphabetical list of countries, but after annexing Korea in 1910, decided to call them something more traditional (not to mention more
Japanese).
The capital of Chosen was Keijo, which is now known as Seoul. Until 1942, Chosen was a colony of Japan: in 1942, it became part of the Japanese Empire alongside Japan. Its subjects were not given Japanese citizenship, but were granted a special status called naikokujin.