Sokui (Enthronement) (即位)
The enthronement means an ascension to the throne, succeeding a monarch (emperor or king) who passed away or abdicated the throne. It is common that the first child or a younger brother of the monarch first becomes a crown prince (dauphin) or kotaitei (an emperor's younger brother considered heir apparent), then succeeds to the throne after the death of the predecessor.
An enthronement is often celebrated with a ceremony called a coronation or an enthronement ceremony. A coronation usually takes place at the time of an enthronement, but there are exceptional cases such as the ceremony is postponed until several years later due to the reason of the domestic political instability.
The enthronement in Japan
In Japan, the enthronement means that an emperor accedes to the throne. It basically refers to the act of the new emperor or empress demonstrating that he or she has succeeded the predecessor by holding the Sokui no rei (ceremony of the enthronement), and there was no distinction between sokui and senso (accession to the throne) in the past. Since the enthronement of the Emperor Kanmu, however, a separation between sokui and senso was created, with senso meaning the succession of Sanshu-no-jingi (Three Imperial Regalia) from the ex-emperor and sokui meaning the new emperor declaring to the people that he has acceded to the throne. Since the two concepts, senso and sokui, are integrated into one in the Imperial House Law and considered as sokui as a whole, no distinction between the two is made today.