noun
demotion from imperial to subject status
Historical term referring to the practice of imperial family members being removed from the imperial family register and becoming commoners. Often used in discussions of the pre-war Imperial Household Law.
戦前の皇室典範では、臣籍降下の規定があった。
The pre-war Imperial Household Law had provisions for demotion from imperial to subject status.
臣籍降嫁 specifically refers to a female imperial family member losing her status upon marriage to a commoner, while 臣籍降下 is a broader term for any imperial family member becoming a subject.
Compound of 臣籍 (imperial subject register) and 降下 (descent, demotion). The term is a technical legal expression from the Meiji era onward.