A mistress is a permanent feminine lover and escort who is not wedded to her partner; the term is employed particularly when her partner is married. The association generally is steady and at least partially-lasting; however, the couple does not live jointly publicly. Also the relation is generally, but not always, covert. There is a conclusion that a mistress may be “kept”— i.e., that the lover is financing some of the woman's living expenditures.
The word mistress was initially employed as a impartial counterpart to mister or master.