Source : Webster's Revised Unabridged Dictionary (1913)

Observer \Ob*serv"er\, n.
   1. One who observes, or pays attention to, anything;
      especially, one engaged in, or trained to habits of, close
      and exact observation; as, an astronomical observer.

            The observed of all observers.        --Shak.

            Careful observers may foretell the hour, By sure
            prognostic, when to dread a shower.   --Swift.

   2. One who keeps any law, custom, regulation, rite, etc.; one
      who conforms to anything in practice. ``Diligent observers
      of old customs.'' --Spenser.

            These . . . hearkend unto observers of times.
                                                  --Deut. xviii.
                                                  14.

   3. One who fulfills or performs; as, an observer of his
      promises.

   4. A sycophantic follower. [Obs.] --Beau. & Fl.

Source : WordNet®

observer
     n 1: a person who becomes aware (of things or events) through the
          senses [syn: {perceiver}, {beholder}]
     2: an expert who observes and comments on something [syn: {commentator}]