mistress
 US /ˈmɪs.trəs/
 UK /ˈmɪs.trəs/

1. 
愛人
a woman in a position of authority or control.
:
• 
she is always mistress of the situation, coolly self-possessed
2. 
愛人
a woman (other than the man's wife) having a sexual relationship with a married man.
:
• 
Elsie knew her husband had a mistress tucked away somewhere