A Milk al-Yameen is a woman with whom a man can have intimate relations without a contract of Nikah. There is no evidence that Prophet Muhammad (sallalahu 'alaihi wa sallam) made a contract of Nikah with Mariyah, the Coptic Christian slave with whom he had a son, Ibrahim. She was not a wife of Nikah, and yet she was his wife; rather she was his Milk al-Yameen!