The Collaborative International Dictionary of English v.0.48:

Lookout \Look"out`\, n.
   1. A careful looking or watching for any object or event.
      [1913 Webster]

   2. The place from which such observation is made.
      [1913 Webster]

   3. A person engaged in watching; a sentinel; a sentry.
      [1913 Webster]

   4. Object or duty of forethought and care; responsibility.
      [Colloq.]
      [1913 Webster]

   on the lookout for in search of; looking for.
      [PJC]

WordNet (r) 3.0 (2006):

lookout
    n 1: a person employed to keep watch for some anticipated event
         [syn: lookout, lookout man, sentinel, sentry,
         watch, spotter, scout, picket]
    2: an elevated post affording a wide view [syn: lookout,
       observation post]
    3: a structure commanding a wide view of its surroundings [syn:
       lookout, observation tower, lookout station,
       observatory]
    4: the act of looking out [syn: lookout, outlook]

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "lookout":
   Argus, Cassandra, Charley, Jeremiah, Texas tower, activities,
   activity, admonisher, affair, affairs, airplane spotter, airscape,
   alert, bag, beacon, belvedere, bleachers, bridge, business,
   cautioner, cityscape, cloudscape, commerce, concern, concernment,
   conning tower, cupola, custodianship, custody, difficulty,
   eagle eye, employ, employment, enterprise, espial, espionage,
   fire patrolman, fire warden, fireguard, flagman, forward observer,
   function, gallery, gazebo, grandstand, guard, guardedness,
   guardianship, headache, interest, invigilation, labor, landscape,
   lighthouse, lighthouse keeper, looking, lookout man, loophole,
   matter, monitor, monitoring, night watchman, note, notice,
   observance, observation, observation post, observatory, occupation,
   outlook, overlook, palaver, panorama, patrol, patroller, patrolman,
   peanut gallery, peeled eye, peephole, perspective, pharos, picket,
   problem, proctoring, prophet of doom, prospect, prudence, qui vive,
   regard, respect, responsibility, ringside, ringside seat,
   riverscape, roundsman, scape, scene, scenery, scenic view, scout,
   seascape, sentinel, sentry, service, sharp eye, sight, sighthole,
   signaler, signalman, skyscape, spotter, spying, stewardship,
   surveillance, sweep, thing, top gallery, tout, tower, townscape,
   undertaking, vedette, view, viewing, vigil, vigilance, vista, ward,
   wariness, warner, watch, watch and ward, watcher, watchful eye,
   watchfulness, watching, watchkeeper, watchman, watchtower,
   waterscape, weather eye, witnessing, work, worry