demonstrator

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that demonstrates, such as a participant in a public display of opinion.
• noun An article or product used in a demonstration.

from The Century Dictionary.

• noun One who points out, exhibits, or explains by examples; specifically, in anatomy, one who exhibits, describes, and explains the parts when dissected; a teacher of practical anatomy.
• noun One who demonstrates; one who proves anything with certainty or with indubitable evidence.
• noun The index finger.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who demonstrates
• noun one who proves anything with certainty, or establishes it by indubitable evidence.
• noun one who shows how a certain device operates or a procedure is performed.
• noun (Anat.) A teacher of practical anatomy.
• noun a person who participates in a demonstration{7}.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun the forefinger
• noun a person involved in a demonstration
• noun an item, (particularly a vehicle), used for customer familiarisation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who demonstrates an article to a prospective buyer
• noun someone who participates in a public display of group feeling
• noun a teacher or teacher's assistant who demonstrates the principles that are being taught