de facto husband; common-law husband
Legal and social term for a man who lives with a woman in a marriage-like relationship without formal registration. Often contrasted with 内縁の妻 (common-law wife).
See also: 内縁の妻 (ないえんのつま)
He has been living with her for over ten years as her common-law husband.
Compound of 内縁 (de facto marriage, informal union) + の (possessive particle) + 夫 (husband).