The Collaborative International Dictionary of English v.0.48:

Demonstrator \Dem"on*stra`tor\ (?; 277), n. [L.: cf. F.
   d['e]monstrateur.]
   1. One who demonstrates; specif.
      (a) one who proves anything with certainty, or establishes
          it by indubitable evidence.
      (b) one who shows how a certain device operates or a
          procedure is performed.
          [1913 Webster]

   2. (Anat.) A teacher of practical anatomy.
      [1913 Webster]

   3. a person who participates in a demonstration[7].
      [PJC]

WordNet (r) 3.0 (2006):

demonstrator
    n 1: a teacher or teacher's assistant who demonstrates the
         principles that are being taught
    2: someone who demonstrates an article to a prospective buyer
       [syn: demonstrator, sales demonstrator]
    3: someone who participates in a public display of group feeling
       [syn: demonstrator, protester]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "demonstrator":
   allegorist, annotator, cicerone, clarifier, commentator, critic,
   cryptanalyst, cryptographer, cryptologist, decoder, definer,
   demythologizer, diaskeuast, dragoman, editor, emendator, emender,
   euhemerist, exegesist, exegete, exegetist, explainer, explicator,
   exponent, expositor, expounder, go-between, guide, hermeneut,
   interpreter, lexicographer, metaphrast, oneirocritic, paraphrast,
   picket, picket line, picketer, scholiast, textual critic,
   translator