Is there a word for a woman's female mistress?

1006a wrote in a comment:

Mistress doesn't imply that the man is married; it implies that the couple are not married to one another, and that the woman is kept by the man, as in paid.

A single man can have a mistress, if he pays most or all of her living expenses in exchange for her sexual availability (and a woman, married or single, could have a mistress (female) or a gigolo (male) if she's paying for a long-term sexual relationship).

The term you want might be the other woman, which definitely implies that the pivot of the triangle is formally committed to one woman while having illicit relations with another.