Source : Webster's Revised Unabridged Dictionary (1913)

Concubinage \Con*cu"bi*nage\, n.
   1. The cohabiting of a man and a woman who are not legally
      married; the state of being a concubine.

   Note: In some countries, concubinage is marriage of an
         inferior kind, or performed with less solemnity than a
         true or formal marriage; or marriage with a woman of
         inferior condition, to whom the husband does not convey
         his rank or quality. Under Roman law, it was the living
         of a man and woman in sexual relations without
         marriage, but in conformity with local law.

   2. (Law) A plea, in which it is alleged that the woman suing
      for dower was not lawfully married to the man in whose
      lands she seeks to be endowed, but that she was his
      concubine.

Source : WordNet®

concubinage
     n : cohabitation without being legally married