also: じょうにん
Refers to a romantic partner, often with a nuance of an extramarital or secret lover. Can be used for both male and female partners, but mistress is a common translation when referring to a woman. Somewhat literary or old-fashioned; in modern casual speech, 恋人 (こいびと) is more common.
He has a wife and children, but apparently he also has a mistress.
When she was young, she was the lover of a certain painter.