Mistress (lover)

A mistress is a man's long term female sexual partner and companion who is not married to him. This term is especially used when the man is married to another woman. The relationship is generally stable; however, the couple do not live together openly. Also, the relationship is usually but not always secret. Rather often the man is paying for some of the woman's living expenses. A mistress has no legal relationship to the man, that makes the difference to the status of a concubine.
The term can also be used to describe the "other" companion in a female same-sex marriage. Likewise, a woman may be married to a man and have a mistress of her own.
Definition.
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".
The historically best known and most researched mistresses are the royal mistresses of European monarchs, for example Madame de Pompadour. 
In the courts of Europe, particularly Versailles and Whitehall in the 17th and 18th centuries, a mistress often had great power and influence. A king might hold numerous mistresses but have a single "favourite mistress" or "official mistress" (in French, "maîtresse en titre"), as with Louis XV and Madame de Pompadour.