Source : Webster's Revised Unabridged Dictionary (1913)

Demonstrator \Dem"on*stra`tor\ (?; 277), n. [L.: cf. F.
   d['e]monstrateur.]
   1. One who demonstrates; one who proves anything with
      certainty, or establishes it by indubitable evidence.

   2. (Anat.) A teacher of practical anatomy.

Source : WordNet®

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: {sales demonstrator}]
     3: someone who participates in a public display of group
        feeling [syn: {protester}]