The Collaborative International Dictionary of English v.0.48:

Visitor \Vis"it*or\ [Cf. F. visiteur.] [Written also visiter.]
   1. One who visits; one who comes or goes to see another, as
      in civility or friendship. "This great flood of visitors."
      --Shak.
      [1913 Webster]

   2. A superior, or a person lawfully appointed for the
      purpose, who makes formal visits of inspection to a
      corporation or an institution. See Visit, v. t., 2, and
      Visitation, n., 2.
      [1913 Webster]

            The king is the visitor of all lay corporations.
                                                  --Blackstone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

visitor
    n 1: someone who visits [syn: visitor, visitant]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "visitor":
   Big Brother, aficionado, arrival, attender, audience, auditor,
   boatswain, boss, buff, caller, chief, comer, company, comptroller,
   controller, entrant, examinant, examiner, fan, floor manager,
   floorman, floorwalker, foreman, freeloader, frequenter, gaffer,
   ganger, gate-crasher, guest, guests, habitue, haunter, head,
   headman, immigrant, in-migrant, incomer, inspector, intruder,
   investigator, monitor, moocher, newcomer, noncommissioned officer,
   observer, overman, overseer, patron, proctor, scrutineer, settler,
   sirdar, slave driver, spectator, straw boss, subforeman, super,
   superintendent, supervisor, surveyor, taskmaster, tester,
   theatergoer, uninvited guest, visitant, visitator