The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
         [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

concubinage
    n 1: cohabitation without being legally married

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

CONCUBINAGE. This term has two different significations; sometimes it means
a species of marriage which took place among the ancients, and which is yet
in use in some countries. In this country it means the act or practice of
cohabiting as man and woman, in sexual commerce, without the authority of
law, or a legal marriage. Vide 1 Bro. Civ. Law, 80; Merl. Rep. b. t.; Dig.
32, 49, 4; Id. 7, 1, 1; Code, 5, 27, 12.