“Where have you been? You've been out gallivanting around with that mistress of yours, haven't you?”
“The mistress of the school has been working hard, and with success. She is intelligent and persevering as a teacher, and kind and attentive to the children.”
“She was appointed as a mistress in charge of a hostel for boarders.”
mistressdom
The realm or sphere of mistresses (female partners in extramarital relationships).