noun
Refers to a romantic partner outside of marriage, often implying an extramarital affair. Can be used for either gender, but 愛人 is more commonly used for a female partner in a heterosexual context. The term carries a somewhat literary or formal tone compared to casual words like 恋人.
彼には妻と子供がいるが、長年愛人もいるらしい。
He has a wife and children, but apparently he's also had a mistress for many years.
社長は愛人と密かに会っていた。
The company president was secretly meeting his lover.