The Collaborative International Dictionary of English v.0.48:

Cohabitation \Co*hab"i*ta"tion\, n. [L. cohabitatio.]
   1. The act or state of dwelling together, or in the same
      place with another. --Feltham.
      [1913 Webster]

   2. (Law) The living together of a man and woman in supposed
      sexual relationship.
      [1913 Webster]

            That the duty of cohabitation is released by the
            cruelty of one of the parties is admitted. --Lord
                                                  Stowell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cohabitation
    n 1: the act of living together and having a sexual relationship
         (especially without being married)