noun
concubine
KAHNG-kyoo-bine/ˈkɑːŋkjubaɪn/
A woman who lives with a man as a sexual partner without the status of a wife.
The married man keeps a concubine in a separate home.

Etymology
“Lying together” supplied the image for cohabitation without marriage. The word also designated a secondary wife of inferior standing, connecting the sexual relationship with a subordinate household status.
In context
They call her a secondary wife, but who doesn’t know that a “co-wife” is really a concubine? This was clearly taking advantage
More examples
- The novel follows a concubine living in the ruler's household.
- She lives with him as his concubine, not his wife.
- The historian studies the lives of the emperor's concubines.
Forms
concubine · concubines
concubinage noun — The state of living as a concubine.