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]