A man who is adopted into his wife's family (a husband adopted as a son-in-law), often to inherit or continue the family line.
長男がいないため、婿養子を迎えて家業を継がせた。
Because there was no eldest son, they adopted a son-in-law to take over the family business.
彼は婿養子に入ることを決め、妻の実家で暮らし始めた。
He decided to be adopted as a son-in-law and started living at his wife's parents' home.