The other woman in an extramarital relationship, generally including sexual relations
A female companion to a master (a man with control, authority or ownership)
A woman, specifically one with great control, authority or ownership
a woman regarded with love and devotion; a sweetheart
A married woman; a wife
A female teacher
A dominatrix
A woman well skilled in anything, or having the mastery over it
To act or take the role of a mistress.
Of a woman: to master; to learn or develop to a high degree of proficiency.