concubine

Definition:

A concubine is a woman who is a secondary wife for a man who already has a wife. Usually a concubine is not legally married to the man.

In the Old Testament, concubines were often female slaves.
A concubine could be acquired by purchase, through military conquest, or in payment of a debt.
For a king, having many concubines was a sign of power.
The New Testament teaches that the practice of having a concubine is against God's will.