noun
common-law wife; de facto wife
Refers to a woman who lives with a man in a marital relationship without being legally married. The term is used in legal and everyday contexts.
彼は内妻と10年以上一緒に暮らしている。
He has been living with his common-law wife for over ten years.
内妻にも相続権が認められる場合がある。
A common-law wife may be granted inheritance rights in some cases.
Compound of 内 (nai, 'inside' or 'unofficial') and 妻 (sai, 'wife'), literally meaning 'unofficial wife'. The term likely emerged in modern Japanese legal and social contexts to describe a woman in a common-law marriage.