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]