A person involved in an illicit sexual relationship or living together without being married; can refer to either a mistress or a male lover.

to live together as a couple without being married, usually implying an illicit or frowned-upon relationship

a woman who lives or has a sexual relationship with a man without being married to him; mistress; kept woman

a woman's lover or illicit partner; paramour

(regional) to live together as husband and wife outside of marriage; to have an illicit affair

To become lovers or have an illicit sexual relationship while not being married.