From Longman Dictionary of Contemporary Englishmistressmis‧tress /ˈmɪstrɪs/ noun [countable] 1 GIRLFRIEND/BOYFRIENDa woman that a man has a sexual relationship with, even though he is married to someone else The prince had shocked society by living openly with his mistress.2 British English old-fashionedSES a female teacher → master the new English mistress3 OWNthe female owner of a dog, horse etc → master4 old-fashionedWORK FOR somebody the female employer of a servant → master The maid looked nervously at her mistress.5 → be (a/the) mistress of something6 → Mistress
Examples from the Corpus
mistress• She thought that her husband had a mistress but could not prove it.• The former prime minister's wife and mistress both attended the funeral.• She reappeared, according to this, in Geneva, where she has made contact with Dimitri Volkov and became his mistress.• He finally married his mistress Kathleen in jail.• At midnight Mahendra would ride to his mistress, stay with her until four and then return home.• I was taken over one by my History mistress.• Harris claims she was the millionaire's mistress.• Official concubines became unofficial mistresses, and mistresses became secrets kept from wives.MistressMistressold useNAME OF A PERSON used with a woman’s family name as a polite way of speaking to her → master → mistressOrigin mistress (1200-1300) Old French maistresse, from maistre; → MASTER1 