Isse ichigen no sei (system of one era per Emperor) (一世一元の制)
Isse ichigen no sei (system of one era per Emperor) refers to the system where an era name will not be changed during the reign of a monarch (Emperor or king).
China
In 1368, Gensho SHU (Chu Yuan-chang), a leader of the Red Turban Rebellion, established a new era named "Kobu" starting with the year when he ascended to the throne in Nanjing City to found the Ming Dynasty, and later introduced a system of one era per Emperor. Before that, an era name had often been changed to radically change public sentiment. The subsequent dynasty Ching took over the system of one era per Emperor from Ming (*However, unlike Japan, where an era name was changed right after monarch replacement, Ming and Ching usually changed the name on New Year's Day of the following year).
In Vietnam, a Sakuho-koku (a kind of vassal state) of Ching, the system of one era per Emperor was introduced when the Nguyễn Dynasty was established in 1802.
Japan
In Japan, the system of one era per Emperor was adopted when the era name "Keio" was changed to "Meiji" in 1868 during the Meiji Restoration although, before that, an era name had sometimes been changed during the reign of an Emperor. The Meiji government renamed the fourth year of Keio to the first year of Meiji and set up Isse ichigen no sei, or the system of one era name per Emperor, by the imperial rescript. Afterwards, the system was formalized by the Era Name Law, which was enacted on June 6, 1979.
This system, however, is used as an excuse for the abolitionism of an era name because it can be regarded as what advocates the monarch's authority.
Korea
In Korea, the system was introduced at the end of the Joseon Dynasty. The government adopted the system, along with the solar calendar, by promulgating the imperial rescript on November 15, 1895 (also can be called the 504th year of "Kaikoku" [an era name of the Joseon Dynasty, meaning establishment of a country.]).