Mistress (lover)
Historically, the term was used for women who could afford a comfortable lifestyle, because a wealthy man financed that so that she will be available for his sexual pleasure. Such a woman could move between the roles of a mistress and a courtesan. Today however, the word mistress is used primarily to refer to the female companion of a man who is married to another woman; in the case of an unmarried man it is usual to speak of a "girlfriend".