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. Today, 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 and emancipated 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.
A mistress is not a prostitute. While a mistress, if “kept”, may essentially be exchanging sex for money, the principal difference is that a mistress keeps herself exclusively reserved for one man, in much the same way as a wife, and there is not so much of a direct quid pro quo between the money and the sex act. There is also usually an emotional and possibly social relationship between a man and his mistress, whereas the relationship to a prostitute is predominantly sexual.
The Fractalius Filter, by Redfield, was used to give the vintage effect. Original photo credit: Valarie Apperson/Talamantes