A mistress is a long-term female lover and companion who is not married to her partner; the term is used especially when her partner is married. The relationship generally is stable and at least semi-permanent; however, the couple does not live together openly. Also the relationship is usually, but not always, secret. There is an implication that a mistress may be "kept"—i.e., that the lover is paying for some of the woman's living expenses.
Historically the term has denoted a "kept woman", who was maintained in a comfortable (or even lavish) lifestyle by a wealthy man so that she will be available for his sexual pleasure. Such a woman could move between the roles of a mistress and a courtesan depending on her situation and environment. In modern times, however, the word "mistress" is used primarily to refer to the female lover of a man who is married to another woman; in the case of an unmarried man, it is usual to speak of a "girlfriend" or "partner." Historically, a man "kept" a mistress. As the term implies, he was responsible for her debts and provided for her in much the same way as he did his wife, although not legally bound to do so. In more recent times, it is more likely that the mistress has a job of her own, and is less, if at all, financially dependent on the man.