a person's (usually a married man’s) mistress is a woman that they have a regular sexual relationship with and who is not their wife

a female teacher in a school, especially a private school

(in the past) the female head of a house, especially one who employed servants

Lizzie, fetch your mistress her shawl.

the female owner of a dog or other animal

a woman who is in a position of authority or control, or who has a lot of skill in something

She wants to be mistress of her own affairs .

She is the mistress of plain speech.

LEARNit Dictionary