1.) The homewrecker who has relations with married or attached men.
2.) A woman with power and authority, having complete control of a household, business establishment and employing help. ‘Mistress of great fortune.’ 'My mistress wouldn't be caught dead in those fugly garments!'
3.) She is skilled in an occupation or art.
4.) The master or owner of animals.
5.) British. A teacher or head school mistress.
6.) In the olden-golden days, you would address an unknown female as mistress. In the same respectable fashion one would use the term, 'miss'.