(1):
(n.) A place or building for making observations on the heavenly bodies.
(2):
(n.) A building fitted with instruments for making systematic observations of any particular class or series of natural phenomena.
(3):
(n.) A place, as an elevated chamber, from which a view may be observed or commanded.
(4):
(n.) A lookout on a flank of a battery whence an officer can note the range and effect of the fire.