The Collaborative International Dictionary of English v.0.48:

Cohabit \Co*hab"it\, v. i. [imp. & p. p. Cohabited; p. pr. &
   vb. n. Cohabiting.] [L. cohabitare; co- + habitare to
   dwell, to have possession of (a place), freg. of habere to
   have. See Habit, n. & v.]
   1. To inhabit or reside in company, or in the same place or
      country.
      [1913 Webster]

            The Philistines were worsted by the captived ark . .
            . : they were not able to cohabit with that holy
            thing.                                --South.
      [1913 Webster]

   2. To dwell or live together as husband and wife.
      [1913 Webster]

            The law presumes that husband and wife cohabit
            together, even after a voluntary separation has
            taken place between them.             --Bouvier.
      [1913 Webster]

   Note: By the common law as existing in the United States,
         marriage is presumed when a man and woman cohabit
         permanently together, being reputed by those who know
         them to be husband and wife, and admitting the
         relationship. --Wharton.
         [1913 Webster]

WordNet (r) 3.0 (2006):

cohabit
    v 1: share living quarters; usually said of people who are not
         married and live together as a couple [syn: cohabit,
         live together, shack up]