say it: MI-struhs
mistress is a noun meaning "an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man". It carries 3 recorded senses.
an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
a woman schoolteacher (especially one regarded as strict)
a woman master who directs the work of others
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for mistress come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.