The Collaborative International Dictionary of English v.0.48:

Concubine \Con"cu*bine\, n. [F., fr. L. concubina; con- + cubare
   to lie down, concumbere to lie together, akin to E. cubit.]
   1. A woman who cohabits with a man without being his wife; a
      paramour.
      [1913 Webster]

   Note: Concubine has been sometimes, but rarely, used of a
         male paramour as well as of a female. --Trench.
         [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

concubine
    n 1: a woman who cohabits with an important man [syn:
         concubine, courtesan, doxy, paramour]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "concubine":
   better half, bondmaid, bondman, bondslave, bondsman, bondswoman,
   captive, chattel, chattel slave, churl, common-law wife,
   debt slave, doxy, feme, feme covert, galley slave, goodwife, goody,
   helot, helpmate, helpmeet, homager, kept mistress, kept woman,
   lady, liege, liege man, liege subject, married woman, matron,
   mistress, odalisque, old lady, old woman, paramour, peon, playmate,
   rib, serf, servant, slave, squaw, subject, theow, thrall,
   unofficial wife, vassal, villein, wedded wife, wife, woman



Easton's 1897 Bible Dictionary:

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).


Bouvier's Law Dictionary, Revised 6th Ed (1856):

CONCUBINE. A woman who cohabits with a man as his wife, without being
married.