The Collaborative International Dictionary of English v.0.48:

Observatory \Ob*serv"a*to*ry\, n.; pl. Observatories. [Cf. F.
   observatoire.]
   1. A place or building for making observations on the
      heavenly bodies.
      [1913 Webster]

            The new observatory in Greenwich Park. --Evelyn.
      [1913 Webster]

   2. A building fitted with instruments for making systematic
      observations of any particular class or series of natural
      phenomena.
      [1913 Webster]

   3. A place, as an elevated chamber, from which a view may be
      observed or commanded.
      [1913 Webster]

   4. (Mil.) A lookout on a flank of a battery whence an officer
      can note the range and effect of the fire; usually
      referred to as an observation post. --Farrow.
      [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Observation post \Ob`ser*va"tion post\ (Mil.)
   a military outpost, usually at a height above the surrounding
   terrain, at or near the front line of a battle from which an
   observer may see the enemy's positions and direct artillery
   fire.
   [PJC]

WordNet (r) 3.0 (2006):

observation post
    n 1: an elevated post affording a wide view [syn: lookout,
         observation post]