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.