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.
He has a wife and children, but apparently he's also had a mistress for many years.
The company president was secretly meeting his lover.
He kept a young woman as his mistress.
He has a mistress even though he's married.
There are rumors that she became the company president's mistress.