Concubine

A concubine is a woman in a relationship with a man with no prospect of marriage.

Jadzia Dax discovered in 2374 that Lady Sirella was descended from a concubine, and did not have as much royal blood as she thought. 

In the mirror universe, the Denobulan $𝜇$ served aboard the as ship's doctor. Phlox turned against $𝜇$ for the prospect of acquiring some concubines.