The Collaborative International Dictionary of English v.0.48:

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.
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            The observed of all observers.        --Shak.
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            Careful observers may foretell the hour,
            By sure prognostic, when to dread a shower. --Swift.
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   2. One who keeps any law, custom, regulation, rite, etc.; one
      who conforms to anything in practice. "Diligent observers
      of old customs." --Spenser.
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            These . . . hearkened unto observers of times.
                                                  --Deut. xviii.
                                                  14.
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   3. One who fulfills or performs; as, an observer of his
      promises.
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   4. A sycophantic follower. [Obs.] --Beau. & Fl.
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WordNet (r) 3.0 (2006):

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