[1] : The Collaborative International Dictionary of English v.0.48

observer \observ"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.
      1913 webster

            the observed of all observers.        --shak.
      1913 webster

            careful observers may foretell the hour,
            by sure prognostic, when to dread a shower. --swift.
      1913 webster

   2. one who keeps any law, custom, regulation, rite, etc.; one
      who conforms to anything in practice. "diligent observers
      of old customs." --spenser.
      1913 webster

            these . . . hearkened unto observers of times.
                                                  --deut. xviii.
                                                  14.
      1913 webster

   3. one who fulfills or performs; as, an observer of his
      promises.
      1913 webster

   4. a sycophantic follower. obs. --beau.  fl.
      1913 webster

[2] : WordNet (r) 2.0

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
see also:
perceiver beholder commentator 

[3] : Moby Thesaurus II by Grady Ward, 1.0

46 moby thesaurus words for "observer":
   tv-viewer, ace, air force, beholder, bird-watcher, bomber pilot,
   bystander, combat pilot, drugstore cowboy, examinant, examiner,
   eyewitness, fighter pilot, flyboy, gaper, gazer, gazer-on,
   girl-watcher, goggler, inspector, investigator, kibitzer, looker,
   looker-on, military pilot, naval pilot, ogler, onlooker, perceiver,
   percipient, scrutineer, seer, sidewalk superintendent, spectator,
   spectatress, spectatrix, suicide pilot, televiewer,
   television-viewer, tester, video-gazer, viewer, visitator, visitor,
   watcher, witness

Observer >> O Words
Observer, definition of term: Observer
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