Styles and titles in Joseon
During the Joseon period, royal titles and styles (forms of address) had been extensive and complex. The general title of the monarch was king (왕; 王; wang) until Gojong crowned himself emperor (황제; 皇帝; hwangje), a title reserved for the Chinese emperor at that time.
Source: Wikipedia — Styles and titles in Joseon (CC BY-SA 4.0)