Refers to a woman financially supported by a man in a long-term extramarital relationship. The term is somewhat dated and may carry a negative or old-fashioned connotation.
He seems to have been keeping a mistress for many years.
In old novels, women living as mistresses often appear.
Compound of 囲い (kakoi, 'enclosure' or 'keeping') + 者 (mono, 'person'). The term literally means 'a person who is kept (in an enclosure)', reflecting the historical practice of a man providing a separate residence for his mistress.