mistress mistress /ˈmɪstrəs/

noun

1. () An adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man  ( fancy woman [Vulgar] , kept woman [Informal] , bit on the side [Informal, British] , mistress )
   an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
     The wealthy businessman had a secret mistress with whom he shared his life.
   The wealthy businessman had a secret mistress with whom he shared his life.
   
   
2. A woman master who directs the work of others  ( mistress )
   a woman master who directs the work of others
   
3. A woman schoolteacher (especially one regarded as strict)  ( mistress , schoolma'am [ N. American] , schoolmarm [ N. American] , schoolmistress )
   a woman schoolteacher (especially one regarded as strict)