A Concubine

1. A sexual partner, especially a woman, to whom one is not or cannot be married.
2. A woman who lives with a man, but who is not a wife.
3. (chiefly historical) A slave-girl or woman, kept for instance in a harem, who is held for sexual service.

(unmarried sexual partner): mistress, sprunk; sexual partner

(woman who lives with a man): cohabitor

(slave-girl): odalisque