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]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "cohabit":
   abide, ball, be intimate, berth, bunk, come together,
   commit adultery, copulate, couple, cover, diddle, domicile,
   domiciliate, doss down, dwell, fornicate, frig, hang out, have sex,
   have sexual relations, inhabit, lay, lie with, live, live together,
   lodge, make it with, make love, make out, mate, mount, nest,
   occupy, perch, remain, reside, room, roost, screw, serve, service,
   sleep with, squat, stay, tenant