Mistress is a women who is with a man who is not her husband.

Mistress 1: a woman who has power, authority, or ownership: as a: the female head of a household b: a woman who employs or supervises servants c: a woman who is in charge of a school or other establishment d: a woman of the Scottish nobility having a status comparable to that of a master2 achiefly British : a female teacher or tutor b: a woman who has achieved mastery in some field3: something personified as female that rules, directs, or dominates 4 a: a woman other than his wife with whom a married man has a continuing sexual relationship Merriam-Webster Online Dictionary

Nymph 1: any of the minor divinities of nature in classical mythology represented as beautiful maidens dwelling in the mountains, forests, trees, and waters Merriam-Webster Online Dictionary