1archaic As a title or polite form of address applied to a woman with the status of mistress. Only with possessive adjective, as "your (also occasionally thy) mistress-ship".
2The position or status of a mistress or female head of a household, etc.; (a period of) female authority as mistress.
3The position or status of a woman who has a long-lasting sexual relationship with a man (especially a married man) who is not her husband.
4rare Also with capital initial. The post of a mistress in a school.
Late Middle English. From mistress + -ship, after mastership.