Koseki, a family register of the Imperial Family (皇籍)
Koseki refers to a family register or eligibility to be a member of the Imperial Family. The antonym of Koseki is Shinminseki or Shinseki, which mean a status of a subject. At present, both Koseki and Shinseki are rarely used as they were the words mainly employed under the Constitution of the Empire of Japan.
Shinseki koka is the term referring to demotion of members of the Imperial family from nobility to subject by marriage with someone with status of a subject. After the enactment of the Japanese Constitution and the Imperial House Act drawn up in accordance with the Constitution in May of 1947, the term Koseki Ridatsu is used to refer to secession from the Imperial Family.