O onun evli metresiydi. - She was his married mistress.
Birinin bir karısı olduğunda, hiçbir şey aynı zamanda bir metres sahibi olmayı engellemez. - Nothing prevents, when one has a wife, to have a mistress at the same time.
a woman who (dishonorably) displaces a wife in the affections of a man; a woman, other than his wife, with whom a married man has a continuing sexual relationship