concubitant

Example:She was his mistress for several years.

Definition:a woman who has a romantic or sexual relationship with a man, usually outside marriage or a formal relationship

Example:They kept their paramour relationship hidden from their spouses.

Definition:a lover who is involved in an extramarital or secret romantic relationship

Example:During ancient times, it was common for rulers to have a concubine.

Definition:a woman who lives with a man without being his legally married wife