[ S or U ] old-fashioned a woman who has control over or responsibility for someone or something:
I'll inform the mistress (of the house) of your arrival, madam.
She intends to remain mistress of (= in charge of) her own life when she gets married.
[ C ] UK old-fashioned a female school teacher:
She got a good report from her German mistress.
[ C ] old-fashioned a female owner of a dog