concubine

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Law A woman who cohabits with a man without being legally married to him.
2. n. In certain societies, such as imperial China, a woman contracted to a man as a secondary wife, often having few legal rights and low social status.

Century Dictionary and Cyclopedia

1. n. A paramour, male or female.
2. n. A wife of inferior condition; one whose relation is in some respects that of a lawful wife, but who has not been united to the husband by the usual ceremonies: as, Hagar and Keturah, the concubines of Abraham. Such concubines were allowed by the Greek and Roman laws, and for many centuries they were more or less tolerated by the church, for both priests and laymen. The concubine of a priest was sometimes called a priestess. See concubinage, 3.
3. n. A woman who cohabits with a man without being married to him; a kept mistress.

Wiktionary

1. n. A woman who lives with a man, but who is not a wife.
2. n. A slave-girl for sexual service prominent in all ancient cultures.
3. n. Signifies a relationship where the male is the dominant partner, socially and economically
4. n. A woman attached to a man solely for reproduction, and who cares for the resulting children without any romantic relationship
5. n. a woman residing in a harem and kept, as by a sultan or an Emperor, for sexual purposes.
6. n. A woman kept by a man who is high in hierarchial society in addition to his wives, e.g in the imperial harem or within a household.

GNU Webster's 1913

1. n. A woman who cohabits with a man without being his wife; a paramour.
2. n. A wife of inferior condition; a lawful wife, but not united to the man by the usual ceremonies, and of inferior condition. Such were Hagar and Keturah, the concubines of Abraham; and such concubines were allowed by the Roman laws. Their children were not heirs of their father.

WordNet 3.0

1. n. a woman who cohabits with an important man