con·cu·bine /ˈkɑŋkjʊˌbaɪn, ˈkɑn-/
  妾,姘婦,第二夫人以下之妻
  Con·cu·bine n.
  1. A woman who cohabits with a man without being his wife; a paramour.
  Note: ☞ Concubine has been sometimes, but rarely, used of a male paramour as well as of a female.
  2. 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.
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  concubine
       n : a woman who cohabits with an important man [syn: courtesan,
            doxy, odalisque, paramour]
  Concubine
     in the Bible denotes a female conjugally united to a man, but in
     a relation inferior to that of a wife. Among the early Jews,
     from various causes, the difference between a wife and a
     concubine was less marked than it would be amongst us. The
     concubine was a wife of secondary rank. There are various laws
     recorded providing for their protection (Ex. 21:7; Deut.
     21:10-14), and setting limits to the relation they sustained to
     the household to which they belonged (Gen. 21:14; 25:6). They
     had no authority in the family, nor could they share in the
     household government.
       The immediate cause of concubinage might be gathered from the
     conjugal histories of Abraham and Jacob (Gen. 16;30). But in
     process of time the custom of concubinage degenerated, and laws
     were made to restrain and regulate it (Ex. 21:7-9).
       Christianity has restored the sacred institution of marriage
     to its original character, and concubinage is ranked with the
     sins of fornication and adultery (Matt. 19:5-9; 1 Cor. 7:2).