mistress definition: 1. a woman who has control over or responsibility for someone or something: 2. a female school teacher: 3. a female owner of a dog. Learn more. 22 Aug Let's be clear. There's never a good excuse for engaging in an affair with someone who is married. But it does happen. And at the end of the day, women who become mistresses or “the other woman” are our daughters, sisters, aunts, mothers, wives, and neighbors. They are women within our communities. Lover is a person who is in romantic relationship with another person. Mistress is a woman who has an extramarital relationship with a married man. Lover is a person who is in romantic relationship with another person. Mistress is a woman who has an extramarital relationship with a married man. A mistress is a relatively long-term female lover and companion who is not married to her partner, especially when her partner is married to someone else. Generally, the relationship is stable and at least semi-permanent, but the couple does not live together openly and the relationship is usually, but not always, secret. Mistress definition, a woman who has authority, control, or power, especially the female head of a household, institution, or other establishment. See more.