What is the Russian word for mistress?

There isn't one single Russian word that perfectly translates to "mistress" in the sense of a romantic relationship outside of marriage. However, here are some options depending on the context:

* Любовница (lyubovnitsa): This is the most common and direct translation, meaning "lover" or "mistress".

* Возлюбленная (vozlyublennaya): This literally means "beloved" and can also be used to refer to a mistress, but it's less direct and can also mean a loved one in general.

* Содержанка (soderzhanka): This term specifically refers to a mistress who is financially supported by a man. It carries a more negative connotation and implies a transactional relationship.

* Пассия (passiya): This word means "passion" or "object of affection" and can sometimes be used to refer to a mistress, although it's less common than the other options.

The best choice ultimately depends on the specific situation and the desired level of formality.