This term refers to the name of a period given by the ruling emperor to indicate and record the years of his period of rule in ancient China. When a new emperor ascended the throne, he would replace the reign title of his predecessor with a new one so as to mark the beginning of a new era. This practice started by Liu Che (156-87 BC), Emperor Wu of the Han Dynasty. He called his reign jian yuan (建元, 140-135 BC). Later on, this practice became a rule. An emperor might also change his reign title if he happened to see something that he believed to be extremely auspicious or in case of an event of great significance. During his reign, an emperor might use one title all through his reign or change it as many as a dozen times. People called the first year of a reign ji yuan (纪元). If the emperor changed his reign title in a certain year, it was called gai yuan (改元). When the Republic of China was founded in 1912, it abolished the reign title practice and began to record the time as the first year of the Republic. The People’s Republic of China, established in 1949, adopted the Christian calendar. Influenced by ancient China, Korea and Vietnam used to have reign titles too. Japan still uses reign title to this day.
Before the reign of Emperor Jing of the Han Dynasty, all emperors denoted the years of their reigns numerically – year one, year two and so on until the end of their lives. When Emperor Wu took the throne, he began to use reign names to designate periods during his reign. He entitled the first period of his reign as jian yuan. (Zhang Shoujie: Annotations on the Records of the Historian)