concubine

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Law A woman who cohabits with a man without being legally married to him.
• noun 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.

from The Century Dictionary.

• noun A paramour, male or female.
• noun 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.
• noun A woman who cohabits with a man without being married to him; a kept mistress.

from the GNU version of the Collaborative International Dictionary of English.

• noun A woman who cohabits with a man without being his wife; a paramour.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A woman who lives with a man, but who is not a wife.
• noun A slave-girl for sexual service prominent in all ancient cultures.
• noun Signifies a relationship where the male is the dominant partner, socially and economically
• noun A woman attached to a man solely for reproduction, and who cares for the resulting children without any romantic relationship.
• noun a woman residing in a harem and kept, as by a sultan or emperor, for sexual purposes.
• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a woman who cohabits with an important man