A Mistress

1. A woman, specifically one with great control, authority or ownership Synonyms: boss (applicable to either sex), head (applicable to either sex), leader (applicable to either sex)male equivalent: master
2. A female teacherSynonym: schoolmarmmale equivalent: master
3. The other woman in an extramarital relationship, generally including sexual relations Synonyms: bit on the side (applicable to either sex), fancy woman, comaré, goomahAntonyms: cicisbeo, fancy man
4. A dominatrix male equivalent: master
5. A woman well skilled in anything, or having the mastery over it
6. a woman regarded with love and devotion; a sweetheart
7. (Scotland) A married woman; a wife
8. (obsolete) The jack in the game of bowls
9. A female companion to a master (a man with control, authority or ownership)
10. female equivalent of master
11. female equivalent of mister

Usage notes

In the extramarital sense, mistress is often narrowly taken to mean a woman involved in a committed extramarital relationship (an affair), often supported financially (a kept woman). It can also be broadly taken to mean a woman involved in an extramarital relationship regardless of the level of commitment, but requires more than a single act of adultery.^[1]